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

[Stephen Smalley]

On Tue, Jul 7, 2026 at 4:01 PM Paul Moore <paul at paul-moore.com> wrote:
>
> 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

They seem to pass for me with and without the patch (they don't
exercise listxattr AFAIK).
This was on the current selinux/dev branch, v7.2-rc1 based.