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 9fcb3d6e1944151936048be791843824a9960378..7c33948361a935ec9a19206565bef4aa431043e1 100644 (file)
--- a/src/backend/storage/buffer/bufmgr.c
+++ b/src/backend/storage/buffer/bufmgr.c
@@ -3013,8 +3013,8 @@ BufferIsPermanent(Buffer buffer)
 XLogRecPtr
 BufferGetLSNAtomic(Buffer buffer)
 {
-       BufferDesc *bufHdr = GetBufferDescriptor(buffer - 1);
        char       *page = BufferGetPage(buffer);
+       BufferDesc *bufHdr;
        XLogRecPtr      lsn;
        uint32          buf_state;
 
@@ -3028,6 +3028,7 @@ BufferGetLSNAtomic(Buffer buffer)
        Assert(BufferIsValid(buffer));
        Assert(BufferIsPinned(buffer));
 
+       bufHdr = GetBufferDescriptor(buffer - 1);
        buf_state = LockBufHdr(bufHdr);
        lsn = PageGetLSN(page);
        UnlockBufHdr(bufHdr, buf_state);