cifs: Fix a kernel BUG with remote OS/2 server (try #3)

commit 6513a81e9325d712f1bfb9a1d7b750134e49ff18 upstream.

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
results in '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.

The workaround is to mask off high 16 bits if the number of bytes written as
returned by the server is greater than the bytes requested by the client as
suggested by Jeff Layton.

Reviewed-by: Jeff Layton <jlayton@samba.org>
Signed-off-by: Suresh Jayaraman <sjayaraman@suse.de>
Signed-off-by: Steve French <sfrench@us.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

diff --git a/fs/cifs/cifssmb.c b/fs/cifs/cifssmb.c
index 941441d3e3860163a51dd5f737cfe9771f283fa9..00a4dbbdfb95ade7f415f91114492d53c9971ac4 100644 (file)
--- a/fs/cifs/cifssmb.c
+++ b/fs/cifs/cifssmb.c
@@ -1517,6 +1517,14 @@ CIFSSMBWrite(const int xid, struct cifsTconInfo *tcon,
                *nbytes = le16_to_cpu(pSMBr->CountHigh);
                *nbytes = (*nbytes) << 16;
                *nbytes += le16_to_cpu(pSMBr->Count);
+
+               /*
+                * Mask off high 16 bits when bytes written as returned by the
+                * server is greater than bytes requested by the client. Some
+                * OS/2 servers are known to set incorrect CountHigh values.
+                */
+               if (*nbytes > count)
+                       *nbytes &= 0xFFFF;
        }
 
        cifs_buf_release(pSMB);
@@ -1605,6 +1613,14 @@ CIFSSMBWrite2(const int xid, struct cifsTconInfo *tcon,
                *nbytes = le16_to_cpu(pSMBr->CountHigh);
                *nbytes = (*nbytes) << 16;
                *nbytes += le16_to_cpu(pSMBr->Count);
+
+               /*
+                * Mask off high 16 bits when bytes written as returned by the
+                * server is greater than bytes requested by the client. OS/2
+                * servers are known to set incorrect CountHigh values.
+                */
+               if (*nbytes > count)
+                       *nbytes &= 0xFFFF;
        }
 
 /*     cifs_small_buf_release(pSMB); */ /* Freed earlier now in SendReceive2 */