Fix unsafe access to BufferDescriptors

Richard Guo · Richard Guo · commit 49a450892a3e · 2025-02-19T11:08:39.000+09:00
When considering a local buffer, the GetBufferDescriptor() call in BufferGetLSNAtomic() would be retrieving a shared buffer with a bad buffer ID. Since the code checks whether the buffer is shared before using the retrieved BufferDesc, this issue did not lead to any malfunction. Nonetheless this seems like trouble waiting to happen, so fix it by ensuring that GetBufferDescriptor() is only called when we know the buffer is shared. Author: Tender Wang <tndrwang@gmail.com> Reviewed-by: Xuneng Zhou <xunengzhou@gmail.com> Reviewed-by: Richard Guo <guofenglinux@gmail.com> Discussion: https://postgr.es/m/CAHewXNku-o46-9cmUgyv6LkSZ25doDrWq32p=oz9kfD8ovVJMg@mail.gmail.com Backpatch-through: 13
diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c
@@ -3973,8 +3973,8 @@ BufferIsPermanent(Buffer buffer)
 XLogRecPtr
 BufferGetLSNAtomic(Buffer buffer)
 {
-	BufferDesc *bufHdr = GetBufferDescriptor(buffer - 1);
 	char	   *page = BufferGetPage(buffer);
+	BufferDesc *bufHdr;
 	XLogRecPtr	lsn;
 	uint32		buf_state;
 
@@ -3988,6 +3988,7 @@ BufferGetLSNAtomic(Buffer buffer)
 	Assert(BufferIsValid(buffer));
 	Assert(BufferIsPinned(buffer));
 
+	bufHdr = GetBufferDescriptor(buffer - 1);
 	buf_state = LockBufHdr(bufHdr);
 	lsn = PageGetLSN(page);
 	UnlockBufHdr(bufHdr, buf_state);
