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

[Paul Moore]

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.

Reviewed-by: Paul Moore <paul at paul-moore.com>

> diff --git a/fs/nfs/nfs4proc.c b/fs/nfs/nfs4proc.c
> index 1360409d8de9..a3415082d610 100644
> --- a/fs/nfs/nfs4proc.c
> +++ b/fs/nfs/nfs4proc.c
> @@ -10585,7 +10585,8 @@ const struct nfs4_minor_version_ops *nfs_v4_minor_ops[] = {
>  static ssize_t nfs4_listxattr(struct dentry *dentry, char *list, size_t size)
>  {
>         ssize_t error, error2, error3;
> -       size_t left = size;
> +       ssize_t left = size;
> +       ssize_t left2;
>
>         error = generic_listxattr(dentry, list, left);
>         if (error < 0)
> @@ -10595,10 +10596,13 @@ static ssize_t nfs4_listxattr(struct dentry *dentry, char *list, size_t size)
>                 left -= error;
>         }
>
> -       error2 = security_inode_listsecurity(d_inode(dentry), &list, &left);
> +       left2 = left;
> +       error2 = security_inode_listsecurity(d_inode(dentry), &list, &left2);
>         if (error2 < 0)
>                 return error2;
> -       error2 = size - error - left;
> +       error2 = left - left2;
> +       if (list)
> +               left -= error2;
>
>         error3 = nfs4_listxattr_nfs4_user(d_inode(dentry), list, left);
>         if (error3 < 0)
>
> base-commit: 6eb8711ece2ce27e52e327a5b7a628ed39b97f45
> --
> 2.55.0

-- 
paul-moore.com