Harden memory context allocators against bogus chunk pointers.

Before commit c6e0fe1f2, functions such as AllocSetFree could pretty
safely presume that they were given a valid chunk pointer for their
own type of context, because the indirect call through a memory
context object and method struct would be very unlikely to work
otherwise.  But now, if pfree() is mistakenly invoked on a pointer
to garbage, we have three chances in eight of ending up at one of
these functions.  That means we need to take extra measures to
verify that we are looking at what we're supposed to be looking at,
especially in debug builds.

Hence, add code to verify that the chunk's back-link to a block header
leads to a memory context object that satisfies the right sort of
IsA() check.  This is still a bit weaker than what we did before,
but for the moment assume that an IsA() check is sufficient.

As a compromise between speed and safety, implement these checks
as Asserts when dealing with small chunks but plain test-and-elogs
when dealing with large (external) chunks.  The latter case should
not be too performance-critical, but the former case probably is.
In slab.c, all chunks are small; but nonetheless use a plain test
in SlabRealloc, because that is certainly not performance-critical,
indeed we should be suspicious that it's being called in error.

In aset.c, additionally add some assertions that the "value" field
of the chunk header is within the small range allowed for freelist
indexes.  Without that, we might find ourselves trying to wipe
most of memory when CLOBBER_FREED_MEMORY is enabled, or scribbling
on a "freelist header" that's far away from the context object.

Eventually, field experience might show us that it's smarter for
these tests to be active always, but for now we'll try to get
away with just having them as assertions.

While at it, also be more uniform about asserting that context
objects passed as parameters are of the type we expect.  Some
places missed that altogether, and slab.c was for no very good
reason doing it differently from the other allocators.

Discussion: https://postgr.es/m/3578387.1665244345@sss.pgh.pa.us

src/backend/utils/mmgr/aset.c

@@ -132,6 +132,10 @@ typedef struct AllocFreeListLink
 #define GetFreeListLink(chkptr) \
 	(AllocFreeListLink *) ((char *) (chkptr) + ALLOC_CHUNKHDRSZ)
 
+/* Validate a freelist index retrieved from a chunk header */
+#define FreeListIdxIsValid(fidx) \
+	((fidx) >= 0 && (fidx) < ALLOCSET_NUM_FREELISTS)
+
 /* Determine the size of the chunk based on the freelist index */
 #define GetChunkSizeFromFreeListIdx(fidx) \
 	((((Size) 1) << ALLOC_MINBITS) << (fidx))
@@ -202,7 +206,15 @@ typedef struct AllocBlockData
  * AllocSetIsValid
  *		True iff set is valid allocation set.
  */
-#define AllocSetIsValid(set) PointerIsValid(set)
+#define AllocSetIsValid(set) \
+	(PointerIsValid(set) && IsA(set, AllocSetContext))
+
+/*
+ * AllocBlockIsValid
+ *		True iff block is valid block of allocation set.
+ */
+#define AllocBlockIsValid(block) \
+	(PointerIsValid(block) && AllocSetIsValid((block)->aset))
 
 /*
  * We always store external chunks on a dedicated block.  This makes fetching
@@ -530,8 +542,7 @@ AllocSetReset(MemoryContext context)
 {
 	AllocSet	set = (AllocSet) context;
 	AllocBlock	block;
-	Size		keepersize PG_USED_FOR_ASSERTS_ONLY
+	Size		keepersize PG_USED_FOR_ASSERTS_ONLY;
-	= set->keeper->endptr - ((char *) set);
 
 	AssertArg(AllocSetIsValid(set));
 
@@ -540,6 +551,9 @@ AllocSetReset(MemoryContext context)
 	AllocSetCheck(context);
 #endif
 
+	/* Remember keeper block size for Assert below */
+	keepersize = set->keeper->endptr - ((char *) set);
+
 	/* Clear chunk freelists */
 	MemSetAligned(set->freelist, 0, sizeof(set->freelist));
 
@@ -598,8 +612,7 @@ AllocSetDelete(MemoryContext context)
 {
 	AllocSet	set = (AllocSet) context;
 	AllocBlock	block = set->blocks;
-	Size		keepersize PG_USED_FOR_ASSERTS_ONLY
+	Size		keepersize PG_USED_FOR_ASSERTS_ONLY;
-	= set->keeper->endptr - ((char *) set);
 
 	AssertArg(AllocSetIsValid(set));
 
@@ -608,6 +621,9 @@ AllocSetDelete(MemoryContext context)
 	AllocSetCheck(context);
 #endif
 
+	/* Remember keeper block size for Assert below */
+	keepersize = set->keeper->endptr - ((char *) set);
+
 	/*
 	 * If the context is a candidate for a freelist, put it into that freelist
 	 * instead of destroying it.
@@ -994,9 +1010,16 @@ AllocSetFree(void *pointer)
 
 	if (MemoryChunkIsExternal(chunk))
 	{
-
+		/* Release single-chunk block. */
 		AllocBlock	block = ExternalChunkGetBlock(chunk);
 
+		/*
+		 * Try to verify that we have a sane block pointer: the block header
+		 * should reference an aset and the freeptr should match the endptr.
+		 */
+		if (!AllocBlockIsValid(block) || block->freeptr != block->endptr)
+			elog(ERROR, "could not find block containing chunk %p", chunk);
+
 		set = block->aset;
 
 #ifdef MEMORY_CONTEXT_CHECKING
@@ -1011,14 +1034,6 @@ AllocSetFree(void *pointer)
 		}
 #endif
 
-
-		/*
-		 * Try to verify that we have a sane block pointer, the freeptr should
-		 * match the endptr.
-		 */
-		if (block->freeptr != block->endptr)
-			elog(ERROR, "could not find block containing chunk %p", chunk);
-
 		/* OK, remove block from aset's list and free it */
 		if (block->prev)
 			block->prev->next = block->next;
@@ -1036,12 +1051,23 @@ AllocSetFree(void *pointer)
 	}
 	else
 	{
-		int			fidx = MemoryChunkGetValue(chunk);
 		AllocBlock	block = MemoryChunkGetBlock(chunk);
-		AllocFreeListLink *link = GetFreeListLink(chunk);
+		int			fidx;
+		AllocFreeListLink *link;
 
+		/*
+		 * In this path, for speed reasons we just Assert that the referenced
+		 * block is good.  We can also Assert that the value field is sane.
+		 * Future field experience may show that these Asserts had better
+		 * become regular runtime test-and-elog checks.
+		 */
+		AssertArg(AllocBlockIsValid(block));
 		set = block->aset;
 
+		fidx = MemoryChunkGetValue(chunk);
+		Assert(FreeListIdxIsValid(fidx));
+		link = GetFreeListLink(chunk);
+
 #ifdef MEMORY_CONTEXT_CHECKING
 		/* Test for someone scribbling on unused space in chunk */
 		if (chunk->requested_size < GetChunkSizeFromFreeListIdx(fidx))
@@ -1089,6 +1115,7 @@ AllocSetRealloc(void *pointer, Size size)
 	AllocSet	set;
 	MemoryChunk *chunk = PointerGetMemoryChunk(pointer);
 	Size		oldsize;
+	int			fidx;
 
 	/* Allow access to private part of chunk header. */
 	VALGRIND_MAKE_MEM_DEFINED(chunk, ALLOCCHUNK_PRIVATE_LEN);
@@ -1105,9 +1132,18 @@ AllocSetRealloc(void *pointer, Size size)
 		Size		oldblksize;
 
 		block = ExternalChunkGetBlock(chunk);
-		oldsize = block->endptr - (char *) pointer;
+
+		/*
+		 * Try to verify that we have a sane block pointer: the block header
+		 * should reference an aset and the freeptr should match the endptr.
+		 */
+		if (!AllocBlockIsValid(block) || block->freeptr != block->endptr)
+			elog(ERROR, "could not find block containing chunk %p", chunk);
+
 		set = block->aset;
 
+		oldsize = block->endptr - (char *) pointer;
+
 #ifdef MEMORY_CONTEXT_CHECKING
 		/* Test for someone scribbling on unused space in chunk */
 		Assert(chunk->requested_size < oldsize);
@@ -1116,13 +1152,6 @@ AllocSetRealloc(void *pointer, Size size)
 				 set->header.name, chunk);
 #endif
 
-		/*
-		 * Try to verify that we have a sane block pointer, the freeptr should
-		 * match the endptr.
-		 */
-		if (block->freeptr != block->endptr)
-			elog(ERROR, "could not find block containing chunk %p", chunk);
-
 #ifdef MEMORY_CONTEXT_CHECKING
 		/* ensure there's always space for the sentinel byte */
 		chksize = MAXALIGN(size + 1);
@@ -1201,9 +1230,20 @@ AllocSetRealloc(void *pointer, Size size)
 	}
 
 	block = MemoryChunkGetBlock(chunk);
-	oldsize = GetChunkSizeFromFreeListIdx(MemoryChunkGetValue(chunk));
+
+	/*
+	 * In this path, for speed reasons we just Assert that the referenced
+	 * block is good. We can also Assert that the value field is sane. Future
+	 * field experience may show that these Asserts had better become regular
+	 * runtime test-and-elog checks.
+	 */
+	AssertArg(AllocBlockIsValid(block));
 	set = block->aset;
 
+	fidx = MemoryChunkGetValue(chunk);
+	Assert(FreeListIdxIsValid(fidx));
+	oldsize = GetChunkSizeFromFreeListIdx(fidx);
+
 #ifdef MEMORY_CONTEXT_CHECKING
 	/* Test for someone scribbling on unused space in chunk */
 	if (chunk->requested_size < oldsize)
@@ -1328,6 +1368,7 @@ AllocSetGetChunkContext(void *pointer)
 	else
 		block = (AllocBlock) MemoryChunkGetBlock(chunk);
 
+	AssertArg(AllocBlockIsValid(block));
 	set = block->aset;
 
 	return &set->header;
@@ -1342,16 +1383,19 @@ Size
 AllocSetGetChunkSpace(void *pointer)
 {
 	MemoryChunk *chunk = PointerGetMemoryChunk(pointer);
+	int			fidx;
 
 	if (MemoryChunkIsExternal(chunk))
 	{
 		AllocBlock	block = ExternalChunkGetBlock(chunk);
 
+		AssertArg(AllocBlockIsValid(block));
 		return block->endptr - (char *) chunk;
 	}
 
-	return GetChunkSizeFromFreeListIdx(MemoryChunkGetValue(chunk)) +
+	fidx = MemoryChunkGetValue(chunk);
-		ALLOC_CHUNKHDRSZ;
+	Assert(FreeListIdxIsValid(fidx));
+	return GetChunkSizeFromFreeListIdx(fidx) + ALLOC_CHUNKHDRSZ;
 }
 
 /*
@@ -1361,6 +1405,8 @@ AllocSetGetChunkSpace(void *pointer)
 bool
 AllocSetIsEmpty(MemoryContext context)
 {
+	AssertArg(AllocSetIsValid(context));
+
 	/*
 	 * For now, we say "empty" only if the context is new or just reset. We
 	 * could examine the freelists to determine if all space has been freed,
@@ -1394,6 +1440,8 @@ AllocSetStats(MemoryContext context,
 	AllocBlock	block;
 	int			fidx;
 
+	AssertArg(AllocSetIsValid(set));
+
 	/* Include context header in totalspace */
 	totalspace = MAXALIGN(sizeof(AllocSetContext));
 
@@ -1405,14 +1453,14 @@ AllocSetStats(MemoryContext context,
 	}
 	for (fidx = 0; fidx < ALLOCSET_NUM_FREELISTS; fidx++)
 	{
+		Size		chksz = GetChunkSizeFromFreeListIdx(fidx);
 		MemoryChunk *chunk = set->freelist[fidx];
 
 		while (chunk != NULL)
 		{
-			Size		chksz = GetChunkSizeFromFreeListIdx(MemoryChunkGetValue(chunk));
 			AllocFreeListLink *link = GetFreeListLink(chunk);
 
-			Assert(GetChunkSizeFromFreeListIdx(fidx) == chksz);
+			Assert(MemoryChunkGetValue(chunk) == fidx);
 
 			freechunks++;
 			freespace += chksz + ALLOC_CHUNKHDRSZ;
@@ -1522,7 +1570,13 @@ AllocSetCheck(MemoryContext context)
 			}
 			else
 			{
-				chsize = GetChunkSizeFromFreeListIdx(MemoryChunkGetValue(chunk));	/* aligned chunk size */
+				int			fidx = MemoryChunkGetValue(chunk);
+
+				if (!FreeListIdxIsValid(fidx))
+					elog(WARNING, "problem in alloc set %s: bad chunk size for chunk %p in block %p",
+						 name, chunk, block);
+
+				chsize = GetChunkSizeFromFreeListIdx(fidx); /* aligned chunk size */
 
 				/*
 				 * Check the stored block offset correctly references this

src/backend/utils/mmgr/generation.c

@@ -105,11 +105,20 @@ struct GenerationBlock
  * simplicity.
  */
 #define GENERATIONCHUNK_PRIVATE_LEN	offsetof(MemoryChunk, hdrmask)
+
 /*
  * GenerationIsValid
- *		True iff set is valid allocation set.
+ *		True iff set is valid generation set.
+ */
+#define GenerationIsValid(set) \
+	(PointerIsValid(set) && IsA(set, GenerationContext))
+
+/*
+ * GenerationBlockIsValid
+ *		True iff block is valid block of generation set.
  */
-#define GenerationIsValid(set) PointerIsValid(set)
+#define GenerationBlockIsValid(block) \
+	(PointerIsValid(block) && GenerationIsValid((block)->context))
 
 /*
  * We always store external chunks on a dedicated block.  This makes fetching
@@ -345,6 +354,8 @@ GenerationAlloc(MemoryContext context, Size size)
 	Size		chunk_size;
 	Size		required_size;
 
+	AssertArg(GenerationIsValid(set));
+
 #ifdef MEMORY_CONTEXT_CHECKING
 	/* ensure there's always space for the sentinel byte */
 	chunk_size = MAXALIGN(size + 1);
@@ -625,6 +636,14 @@ GenerationFree(void *pointer)
 	if (MemoryChunkIsExternal(chunk))
 	{
 		block = ExternalChunkGetBlock(chunk);
+
+		/*
+		 * Try to verify that we have a sane block pointer: the block header
+		 * should reference a generation context.
+		 */
+		if (!GenerationBlockIsValid(block))
+			elog(ERROR, "could not find block containing chunk %p", chunk);
+
 #if defined(MEMORY_CONTEXT_CHECKING) || defined(CLOBBER_FREED_MEMORY)
 		chunksize = block->endptr - (char *) pointer;
 #endif
@@ -632,6 +651,14 @@ GenerationFree(void *pointer)
 	else
 	{
 		block = MemoryChunkGetBlock(chunk);
+
+		/*
+		 * In this path, for speed reasons we just Assert that the referenced
+		 * block is good.  Future field experience may show that this Assert
+		 * had better become a regular runtime test-and-elog check.
+		 */
+		AssertArg(GenerationBlockIsValid(block));
+
 #if defined(MEMORY_CONTEXT_CHECKING) || defined(CLOBBER_FREED_MEMORY)
 		chunksize = MemoryChunkGetValue(chunk);
 #endif
@@ -723,11 +750,27 @@ GenerationRealloc(void *pointer, Size size)
 	if (MemoryChunkIsExternal(chunk))
 	{
 		block = ExternalChunkGetBlock(chunk);
+
+		/*
+		 * Try to verify that we have a sane block pointer: the block header
+		 * should reference a generation context.
+		 */
+		if (!GenerationBlockIsValid(block))
+			elog(ERROR, "could not find block containing chunk %p", chunk);
+
 		oldsize = block->endptr - (char *) pointer;
 	}
 	else
 	{
 		block = MemoryChunkGetBlock(chunk);
+
+		/*
+		 * In this path, for speed reasons we just Assert that the referenced
+		 * block is good.  Future field experience may show that this Assert
+		 * had better become a regular runtime test-and-elog check.
+		 */
+		AssertArg(GenerationBlockIsValid(block));
+
 		oldsize = MemoryChunkGetValue(chunk);
 	}
 
@@ -845,6 +888,7 @@ GenerationGetChunkContext(void *pointer)
 	else
 		block = (GenerationBlock *) MemoryChunkGetBlock(chunk);
 
+	AssertArg(GenerationBlockIsValid(block));
 	return &block->context->header;
 }
 
@@ -863,6 +907,7 @@ GenerationGetChunkSpace(void *pointer)
 	{
 		GenerationBlock *block = ExternalChunkGetBlock(chunk);
 
+		AssertArg(GenerationBlockIsValid(block));
 		chunksize = block->endptr - (char *) pointer;
 	}
 	else
@@ -881,6 +926,8 @@ GenerationIsEmpty(MemoryContext context)
 	GenerationContext *set = (GenerationContext *) context;
 	dlist_iter	iter;
 
+	AssertArg(GenerationIsValid(set));
+
 	dlist_foreach(iter, &set->blocks)
 	{
 		GenerationBlock *block = dlist_container(GenerationBlock, node, iter.cur);
@@ -917,6 +964,8 @@ GenerationStats(MemoryContext context,
 	Size		freespace = 0;
 	dlist_iter	iter;
 
+	AssertArg(GenerationIsValid(set));
+
 	/* Include context header in totalspace */
 	totalspace = MAXALIGN(sizeof(GenerationContext));
 

src/backend/utils/mmgr/slab.c

@@ -111,6 +111,21 @@ typedef struct SlabBlock
 #define SlabChunkIndex(slab, block, chunk)	\
 	(((char *) chunk - SlabBlockStart(block)) / slab->fullChunkSize)
 
+/*
+ * SlabIsValid
+ *		True iff set is valid slab allocation set.
+ */
+#define SlabIsValid(set) \
+	(PointerIsValid(set) && IsA(set, SlabContext))
+
+/*
+ * SlabBlockIsValid
+ *		True iff block is valid block of slab allocation set.
+ */
+#define SlabBlockIsValid(block) \
+	(PointerIsValid(block) && SlabIsValid((block)->slab))
+
+
 /*
  * SlabContextCreate
  *		Create a new Slab context.
@@ -236,10 +251,10 @@ SlabContextCreate(MemoryContext parent,
 void
 SlabReset(MemoryContext context)
 {
+	SlabContext *slab = (SlabContext *) context;
 	int			i;
-	SlabContext *slab = castNode(SlabContext, context);
 
-	Assert(slab);
+	AssertArg(SlabIsValid(slab));
 
 #ifdef MEMORY_CONTEXT_CHECKING
 	/* Check for corruption and leaks before freeing */
@@ -293,12 +308,12 @@ SlabDelete(MemoryContext context)
 void *
 SlabAlloc(MemoryContext context, Size size)
 {
-	SlabContext *slab = castNode(SlabContext, context);
+	SlabContext *slab = (SlabContext *) context;
 	SlabBlock  *block;
 	MemoryChunk *chunk;
 	int			idx;
 
-	Assert(slab);
+	AssertArg(SlabIsValid(slab));
 
 	Assert((slab->minFreeChunks >= 0) &&
 		   (slab->minFreeChunks < slab->chunksPerBlock));
@@ -450,10 +465,18 @@ SlabAlloc(MemoryContext context, Size size)
 void
 SlabFree(void *pointer)
 {
-	int			idx;
 	MemoryChunk *chunk = PointerGetMemoryChunk(pointer);
 	SlabBlock  *block = MemoryChunkGetBlock(chunk);
-	SlabContext *slab = block->slab;
+	SlabContext *slab;
+	int			idx;
+
+	/*
+	 * For speed reasons we just Assert that the referenced block is good.
+	 * Future field experience may show that this Assert had better become a
+	 * regular runtime test-and-elog check.
+	 */
+	AssertArg(SlabBlockIsValid(block));
+	slab = block->slab;
 
 #ifdef MEMORY_CONTEXT_CHECKING
 	/* Test for someone scribbling on unused space in chunk */
@@ -540,9 +563,18 @@ SlabRealloc(void *pointer, Size size)
 {
 	MemoryChunk *chunk = PointerGetMemoryChunk(pointer);
 	SlabBlock  *block = MemoryChunkGetBlock(chunk);
-	SlabContext *slab = block->slab;
+	SlabContext *slab;
+
+	/*
+	 * Try to verify that we have a sane block pointer: the block header
+	 * should reference a slab context.  (We use a test-and-elog, not just
+	 * Assert, because it seems highly likely that we're here in error in the
+	 * first place.)
+	 */
+	if (!SlabBlockIsValid(block))
+		elog(ERROR, "could not find block containing chunk %p", chunk);
+	slab = block->slab;
 
-	Assert(slab);
 	/* can't do actual realloc with slab, but let's try to be gentle */
 	if (size == slab->chunkSize)
 		return pointer;
@@ -560,11 +592,9 @@ SlabGetChunkContext(void *pointer)
 {
 	MemoryChunk *chunk = PointerGetMemoryChunk(pointer);
 	SlabBlock  *block = MemoryChunkGetBlock(chunk);
-	SlabContext *slab = block->slab;
-
-	Assert(slab != NULL);
 
-	return &slab->header;
+	AssertArg(SlabBlockIsValid(block));
+	return &block->slab->header;
 }
 
 /*
@@ -577,9 +607,10 @@ SlabGetChunkSpace(void *pointer)
 {
 	MemoryChunk *chunk = PointerGetMemoryChunk(pointer);
 	SlabBlock  *block = MemoryChunkGetBlock(chunk);
-	SlabContext *slab = block->slab;
+	SlabContext *slab;
 
-	Assert(slab);
+	AssertArg(SlabBlockIsValid(block));
+	slab = block->slab;
 
 	return slab->fullChunkSize;
 }
@@ -591,9 +622,9 @@ SlabGetChunkSpace(void *pointer)
 bool
 SlabIsEmpty(MemoryContext context)
 {
-	SlabContext *slab = castNode(SlabContext, context);
+	SlabContext *slab = (SlabContext *) context;
 
-	Assert(slab);
+	AssertArg(SlabIsValid(slab));
 
 	return (slab->nblocks == 0);
 }
@@ -613,13 +644,15 @@ SlabStats(MemoryContext context,
 		  MemoryContextCounters *totals,
 		  bool print_to_stderr)
 {
-	SlabContext *slab = castNode(SlabContext, context);
+	SlabContext *slab = (SlabContext *) context;
 	Size		nblocks = 0;
 	Size		freechunks = 0;
 	Size		totalspace;
 	Size		freespace = 0;
 	int			i;
 
+	AssertArg(SlabIsValid(slab));
+
 	/* Include context header in totalspace */
 	totalspace = slab->headerSize;
 
@@ -672,11 +705,11 @@ SlabStats(MemoryContext context,
 void
 SlabCheck(MemoryContext context)
 {
+	SlabContext *slab = (SlabContext *) context;
 	int			i;
-	SlabContext *slab = castNode(SlabContext, context);
 	const char *name = slab->header.name;
 
-	Assert(slab);
+	AssertArg(SlabIsValid(slab));
 	Assert(slab->chunksPerBlock > 0);
 
 	/* walk all the freelists */