[PATCH v2] NFSv4.2: fix nfs4_listxattr size accounting

[Paul Moore]

On Tue, Jul 7, 2026 at 3:12 PM Anna Schumaker <anna at kernel.org> wrote:
> On Tue, Jul 7, 2026, at 2:48 PM, Paul Moore wrote:
> > On Tue, Jul 7, 2026 at 11:24 AM Achilles Gaikwad
> > <achillesgaikwad at gmail.com> wrote:
> >>
> >> A call to listxattr() with a buffer size of 0 returns the actual
> >> size of the buffer needed for a subsequent call. On an NFSv4.2
> >> mount this triggers the following oops:
> >>
> >>   [  399.768687] BUG: kernel NULL pointer dereference, address: 0000000000000000
> >>   [  399.768705] RIP: 0010:_copy_from_pages+0x44/0xe0
> >>   [  399.768722] Call Trace:
> >>   [  399.768723]  nfs4_xattr_alloc_entry+0x1bf/0x1e0
> >>   [  399.768730]  nfs4_xattr_cache_set_list+0x43/0x1f0
> >>   [  399.768731]  nfs4_listxattr+0x21f/0x250
> >>   [  399.768733]  vfs_listxattr+0x55/0xa0
> >>   [  399.768736]  listxattr+0x23/0x160
> >>   [  399.768737]  path_listxattrat+0xba/0x1e0
> >>   [  399.768739]  do_syscall_64+0xe2/0x680
> >>
> >> security_inode_listsecurity() (via the xattr_list_one() helper) now
> >> decrements the remaining size even when the buffer pointer is NULL, so
> >> in the size-query case, 'left' underflows to a huge size_t value. As a
> >> result, nfs4_listxattr_nfs4_user() treats the NULL buffer as a real one,
> >> leading to a NULL pointer dereference in _copy_from_pages().
> >>
> >> security_inode_listsecurity() does not return the number of bytes
> >> it added to the list, so the code derived it as
> >> 'size - error - left'. That is also wrong in the size-query case:
> >> the generic_listxattr() contribution is only subtracted from 'left'
> >> when a buffer is present. Thus, the query result comes up short by
> >> exactly that contribution (e.g., "system.nfs4_acl" on a mount with
> >> ACL support), and a caller that allocates the returned size gets
> >> -ERANGE on the subsequent call.
> >>
> >> Declare 'left' as ssize_t, use a scratch copy to measure security
> >> hook consumption, and only decrement 'left' if a buffer is present.
> >>
> >> Fixes: f71ece9712b7 ("security,fs,nfs,net: update security_inode_listsecurity() interface")
> >> Suggested-by: Paul Moore <paul at paul-moore.com>
> >> Signed-off-by: Achilles Gaikwad <achillesgaikwad at gmail.com>
> >> ---
> >> Changes in v2:
> >>  - Use a scratch variable to track security label size directly,
> >>    replacing the old formula that undercounted the size-query case.
> >>  - Drop the now-unneeded NULL-buffer special case for
> >>    nfs4_listxattr_nfs4_user().
> >>  - Retitled from "fix nfs4_listxattr NULL pointer dereference"
> >>    (the same accounting bug caused both the oops and the undercount).
> >> v1: https://lore.kernel.org/linux-nfs/20260703102759.9626-1-achillesgaikwad@gmail.com/
> >>  fs/nfs/nfs4proc.c | 10 +++++++---
> >>  1 file changed, 7 insertions(+), 3 deletions(-)
> >
> > [CC'd the LSM and SELinux lists for visibility]
> >
> > Unfortunately my testing was unsuccessful due to an NFS problem that
> > started with the v7.2 merge window that I haven't had the time to
> > bisect yet.  Assuming the NFS folks are okay with this change, I
> > figure they will want to send it up to Linus via their tree, if not
> > let me know and I can send this up via the LSM tree.
>
> Yeah, we'll send it through the NFS tree.

Thanks Anna.

> I'll be curious to hear
> what problem you're hitting, and what patch is the culprit once you
> do that bisect!

Yes, me too :)

I'm still working through a review backlog so it might be a bit before
I have a chance, but in case anyone wants to test it out, it's easily
reproduced using the selinux-testsuite and the NFS tests:

https://github.com/SELinuxProject/selinux-testsuite#nfs

-- 
paul-moore.com