[PATCH] ovl: Check for NULL OVL_E() results

[Kees Cook <kees@kernel.org>]

GCC notices that it is possible for OVL_E() to return NULL (which
implies that d_inode(dentry) may be NULL). This would result in out
of bounds reads via container_of(), seen with GCC 15's -Warray-bounds
-fdiagnostics-details. For example:

In file included from ./arch/x86/include/generated/asm/rwonce.h:1,
                 from ../include/linux/compiler.h:339,
                 from ../include/linux/export.h:5,
                 from ../include/linux/linkage.h:7,
                 from ../include/linux/fs.h:5,
                 from ../fs/overlayfs/util.c:7:
In function 'ovl_upperdentry_dereference',
    inlined from 'ovl_dentry_upper' at ../fs/overlayfs/util.c:305:9,
    inlined from 'ovl_path_type' at ../fs/overlayfs/util.c:216:6:
../include/asm-generic/rwonce.h:44:26: error: array subscript 0 is outside array bounds of 'struct inode[7486503276667837]' [-Werror=array-bounds=]
   44 | #define __READ_ONCE(x)  (*(const volatile __unqual_scalar_typeof(x) *)&(x))                       |                         ~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~                 ../include/asm-generic/rwonce.h:50:9: note: in expansion of macro '__READ_ONCE'
   50 |         __READ_ONCE(x);                                                 \
      |         ^~~~~~~~~~~
../fs/overlayfs/ovl_entry.h:195:16: note: in expansion of macro 'READ_ONCE'                           195 |         return READ_ONCE(oi->__upperdentry);
      |                ^~~~~~~~~
  'ovl_path_type': event 1
  185 |         return inode ? OVL_I(inode)->oe : NULL;
  'ovl_path_type': event 2

Explicitly check the result of OVL_E() and return accordingly.

Signed-off-by: Kees Cook <kees@kernel.org>
---
Cc: Miklos Szeredi <miklos@szeredi.hu>
Cc: Amir Goldstein <amir73il@gmail.com>
Cc: linux-unionfs@vger.kernel.org
---
 fs/overlayfs/util.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/fs/overlayfs/util.c b/fs/overlayfs/util.c
index 3bb107471fb4..32ec5eec32fa 100644
--- a/fs/overlayfs/util.c
+++ b/fs/overlayfs/util.c
@@ -213,6 +213,9 @@ enum ovl_path_type ovl_path_type(struct dentry *dentry)
 	struct ovl_entry *oe = OVL_E(dentry);
 	enum ovl_path_type type = 0;
 
+	if (WARN_ON_ONCE(oe == NULL))
+		return 0;
+
 	if (ovl_dentry_upper(dentry)) {
 		type = __OVL_PATH_UPPER;
 
@@ -1312,6 +1315,9 @@ bool ovl_is_metacopy_dentry(struct dentry *dentry)
 {
 	struct ovl_entry *oe = OVL_E(dentry);
 
+	if (WARN_ON_ONCE(oe == NULL))
+		return false;
+
 	if (!d_is_reg(dentry))
 		return false;
 
-- 
2.34.1