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

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 <[email protected]>
Signed-off-by: Achilles Gaikwad <[email protected]>
---
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-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org/
 fs/nfs/nfs4proc.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

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