[ Upstream commit aa7aeee169480e98cf41d83c01290a37e569be6d ]

Ensure that the fid's iounit field is set to zero when a new fid is created. Certain 9P operations, such as OPEN and CREATE, allow the server to reply with an iounit size which the client code assigns to the p9_fid struct shortly after the fid is created by p9_fid_create(). On the other hand, an XATTRWALK operation doesn't allow for the server to specify an iounit value. The iounit field of the newly allocated p9_fid struct remained uninitialized in that case. Depending on allocation patterns, the iounit value could have been something reasonable that was carried over from previously freed fids or, in the worst case, could have been arbitrary values from non-fid related usages of the memory location.

The bug was detected in the Windows Subsystem for Linux 2 (WSL2) kernel after the uninitialized iounit field resulted in the typical sequence of two getxattr(2) syscalls, one to get the size of an xattr and another after allocating a sufficiently sized buffer to fit the xattr value, to hit an unexpected ERANGE error in the second call to getxattr(2). An uninitialized iounit field would sometimes force rsize to be smaller than the xattr value size in p9_client_read_once() and the 9P server in WSL refused to chunk up the READ on the attr_fid and, instead, returned ERANGE to the client. The virtfs server in QEMU seems happy to chunk up the READ and this problem goes undetected there.

Link: https://lkml.kernel.org/r/20220710141402.803295-1-tyhicks@linux.microsoft.co... Fixes: ebf46264a004 ("fs/9p: Add support user. xattr") Cc: stable@vger.kernel.org Signed-off-by: Tyler Hicks tyhicks@linux.microsoft.com Reviewed-by: Christian Schoenebeck linux_oss@crudebyte.com Signed-off-by: Dominique Martinet asmadeus@codewreck.org [tyhicks: Adjusted context due to: - Lack of fid refcounting introduced in v5.11 commit 6636b6dcc3db ("9p: add refcount to p9_fid struct") - Difference in how buffer sizes are specified v5.16 commit 6e195b0f7c8e ("9p: fix a bunch of checkpatch warnings")] Signed-off-by: Tyler Hicks tyhicks@linux.microsoft.com ---

Tested on top of v5.10.136. Verified to apply to v5.4 and v4.19 and the resulting code was read for correctness.

Tyler

net/9p/client.c | 5 +---- 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/net/9p/client.c b/net/9p/client.c index bf6ed00d7c37..e8862cd4f91b 100644 --- a/net/9p/client.c +++ b/net/9p/client.c @@ -893,16 +893,13 @@ static struct p9_fid *p9_fid_create(struct p9_client *clnt) struct p9_fid *fid;

p9_debug(P9_DEBUG_FID, "clnt %p\n", clnt); - fid = kmalloc(sizeof(struct p9_fid), GFP_KERNEL); + fid = kzalloc(sizeof(struct p9_fid), GFP_KERNEL); if (!fid) return NULL;

- memset(&fid->qid, 0, sizeof(struct p9_qid)); fid->mode = -1; fid->uid = current_fsuid(); fid->clnt = clnt; - fid->rdir = NULL; - fid->fid = 0;

idr_preload(GFP_KERNEL); spin_lock_irq(&clnt->lock);