Quoting Eddie.Horng (eddie.horng@mediatek.com):

The code in cap_inode_getsecurity(), introduced by commit 8db6c34f1dbc ("Introduce v3 namespaced file capabilities"), should use d_find_any_alias() instead of d_find_alias() do handle unhashed dentry correctly. This is needed, for example, if execveat() is called with an open but unlinked overlayfs file, because overlayfs unhashes dentry on unlink. This is a regression of real life application, first reported at https://www.spinics.net/lists/linux-unionfs/msg05363.html

Below reproducer and setup can reproduce the case. const char* exec="echo"; const char *newargv[] = { "echo", "hello", NULL}; const char *newenviron[] = { NULL }; int fd, err;

fd = open(exec, O_PATH); unlink(exec); err = syscall(322/*SYS_execveat*/, fd, "", newargv, newenviron, AT_EMPTY_PATH); if(err<0) fprintf(stderr, "execveat: %s\n", strerror(errno));

gcc compile into ~/test/a.out mount -t overlay -orw,lowerdir=/mnt/l,upperdir=/mnt/u,workdir=/mnt/w none /mnt/m cd /mnt/m cp /bin/echo . ~/test/a.out

Expected result: hello Actually result: execveat: Invalid argument dmesg: Invalid argument reading file caps for /dev/fd/3

The 2nd reproducer and setup emulates similar case but for regular filesystem: const char* exec="echo"; int fd, err; char buf[256];

fd = open(exec, O_RDONLY); unlink(exec); err = fgetxattr(fd, "security.capability", buf, 256); if(err<0) fprintf(stderr, "fgetxattr: %s\n", strerror(errno));

gcc compile into ~/test_fgetxattr

cd /tmp cp /bin/echo . ~/test_fgetxattr

Result: fgetxattr: Invalid argument

On regular filesystem, for example, ext4 read xattr from disk and return to execveat(), will not trigger this issue, however, the overlay attr handler pass real dentry to vfs_getxattr() will. This reproducer calls fgetxattr() with an unlinked fd, involkes vfs_getxattr() then reproduced the case that d_find_alias() in cap_inode_getsecurity() can't find the unlinked dentry.

Suggested-by: Amir Goldstein amir73il@gmail.com Acked-by: Amir Goldstein amir73il@gmail.com Acked-by: Serge E. Hallyn serge@hallyn.com Fixes: 8db6c34f1dbc ("Introduce v3 namespaced file capabilities") Cc: stable@vger.kernel.org # v4.14 Signed-off-by: Eddie Horng eddie.horng@mediatek.com

Hey Eric,

if the patch looks ok to you, do you mind pulling it in through your tree?

thanks, -serge

---

Changes in v2:

• fix commit message wrapped at 74 chars
• added previous acked-by

---

Changes in v3:

• added original case report link
• added 2nd reproducer for regular filesystems
• added acked-by Serge E. Hallyn
• add Cc

---

security/commoncap.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/security/commoncap.c b/security/commoncap.c index 1ce701fcb3f3..147f6131842a 100644 --- a/security/commoncap.c +++ b/security/commoncap.c @@ -388,7 +388,7 @@ int cap_inode_getsecurity(struct inode *inode, const char *name, void **buffer, if (strcmp(name, "capability") != 0) return -EOPNOTSUPP;

• dentry = d_find_alias(inode);

• dentry = d_find_any_alias(inode); if (!dentry) return -EINVAL;

2.12.5