Fix unsafe access to BufferDescriptors

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
index 6181673095572a324c7354d672755edca796e7b9..f8d30bf71e1a87c21e3a9777fc2f6df653cc8eaa 100644 (file)
--- 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);