[linux-cifs-client] [PATCH] cifs: Fix a kernel BUG with remote OS/2 server

[Suresh Jayaraman]

Sorry about one more iteration -- fixed a few checkpatch warnings.

While chasing a bug report involving a OS/2 server, I noticed the server sets
pSMBr->CountHigh to a incorrect value even in case of normal writes. This
leads to 'nbytes' being computed wrongly and triggers a kernel BUG at
mm/filemap.c.

void iov_iter_advance(struct iov_iter *i, size_t bytes)
{
        BUG_ON(i->count < bytes);    <--- BUG here

Why the server is setting 'CountHigh' is not clear but only does so after
writing 64k bytes. Though this looks like the server bug, the client side
crash may not be acceptable.

When the write returns successfully and the bytes written as returned by
the server is greater than bytes requested by the client, reset it to original
bytes requested.


Cc: Jeff Layton <jlayton at samba.org>
Signed-off-by: Suresh Jayaraman <sjayaraman at suse.de>
---
 fs/cifs/cifssmb.c |   20 ++++++++++++++++++++
 1 files changed, 20 insertions(+), 0 deletions(-)

diff --git a/fs/cifs/cifssmb.c b/fs/cifs/cifssmb.c
index 7cc7f83..5872e58 100644
--- a/fs/cifs/cifssmb.c
+++ b/fs/cifs/cifssmb.c
@@ -1517,6 +1517,16 @@ CIFSSMBWrite(const int xid, struct cifsTconInfo *tcon,
 		*nbytes = le16_to_cpu(pSMBr->CountHigh);
 		*nbytes = (*nbytes) << 16;
 		*nbytes += le16_to_cpu(pSMBr->Count);
+
+		/*
+		 * Workaround: Some legacy servers (read OS/2) might incorrectly
+		 * set CountHigh for normal writes resulting in wrong 'nbytes'
+		 * value. So when the write returns successfully and the bytes
+		 * written as returned by the server is greater than bytes
+		 * requested, reset it to original bytes requested.
+		 */
+		if (*nbytes > count)
+			*nbytes = count;
 	}
 
 	cifs_buf_release(pSMB);
@@ -1605,6 +1615,16 @@ CIFSSMBWrite2(const int xid, struct cifsTconInfo *tcon,
 		*nbytes = le16_to_cpu(pSMBr->CountHigh);
 		*nbytes = (*nbytes) << 16;
 		*nbytes += le16_to_cpu(pSMBr->Count);
+
+		/* Workaround: Some legacy servers (read OS/2) might incorrectly
+		 * set CountHigh for normal writes resulting in wrong 'nbytes'
+		 * value. So when the write returns successfully and the bytes
+		 * written as returned by the server is greater than bytes
+		 * requested, reset it to original bytes requested.
+		 */
+
+		if (*nbytes > count)
+			*nbytes = count;
 	}
 
 /*	cifs_small_buf_release(pSMB); */ /* Freed earlier now in SendReceive2 */