openat2: reject RESOLVE_BENEATH|RESOLVE_IN_ROOT

This was an oversight in the original implementation, as it makes no
sense to specify both scoping flags to the same openat2(2) invocation
(before this patch, the result of such an invocation was equivalent to
RESOLVE_IN_ROOT being ignored).

This is a userspace-visible ABI change, but the only user of openat2(2)
at the moment is LXC which doesn't specify both flags and so no
userspace programs will break as a result.

Fixes: fddb5d430ad9 ("open: introduce openat2(2) syscall")
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Cc: <stable@vger.kernel.org> # v5.6+
Link: https://lore.kernel.org/r/20201027235044.5240-2-cyphar@cyphar.com
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

diff --git a/fs/open.c b/fs/open.c
index 9af548fb841b00aa5f5c9eb34f7e36150e090cfd..4d7537ae59df5045c3548fabd9c28b0870d4841c 100644 (file)
--- a/fs/open.c
+++ b/fs/open.c
@@ -1010,6 +1010,10 @@ inline int build_open_flags(const struct open_how *how, struct open_flags *op)
        if (how->resolve & ~VALID_RESOLVE_FLAGS)
                return -EINVAL;
 
+       /* Scoping flags are mutually exclusive. */
+       if ((how->resolve & RESOLVE_BENEATH) && (how->resolve & RESOLVE_IN_ROOT))
+               return -EINVAL;
+
        /* Deal with the mode. */
        if (WILL_CREATE(flags)) {
                if (how->mode & ~S_IALLUGO)