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 adb39dada4caf9cfd11a0452ec34ab36b3bedbbb..79e6d6fc3d5d00b2882f6c97718705159ed1c076 100644 (file)
--- a/src/backend/storage/buffer/bufmgr.c
+++ b/src/backend/storage/buffer/bufmgr.c
@@ -3030,8 +3030,8 @@ BufferIsPermanent(Buffer buffer)
 XLogRecPtr
 BufferGetLSNAtomic(Buffer buffer)
 {
-   BufferDesc *bufHdr = GetBufferDescriptor(buffer - 1);
    char       *page = BufferGetPage(buffer);
+   BufferDesc *bufHdr;
    XLogRecPtr  lsn;
    uint32      buf_state;
 
@@ -3045,6 +3045,7 @@ BufferGetLSNAtomic(Buffer buffer)
    Assert(BufferIsValid(buffer));
    Assert(BufferIsPinned(buffer));
 
+   bufHdr = GetBufferDescriptor(buffer - 1);
    buf_state = LockBufHdr(bufHdr);
    lsn = PageGetLSN(page);
    UnlockBufHdr(bufHdr, buf_state);