bpf: Add sanity check for upper ptr_limit

BugLink: https://bugs.launchpad.net/bugs/1921710
commit 1b1597e64e1a610c7a96710fc4717158e98a08b3 upstream.

Given we know the max possible value of ptr_limit at the time of retrieving
the latter, add basic assertions, so that the verifier can bail out if
anything looks odd and reject the program. Nothing triggered this so far,
but it also does not hurt to have these.

Signed-off-by: Piotr Krysiuk <piotras@gmail.com>
Co-developed-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 78abf6cc6a1abc171dbeca2cf76b93662da911c8..ab23dfb9df1b191f380d54689c1f65f5acf93c40 100644 (file)
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -5389,10 +5389,14 @@ static int retrieve_ptr_limit(const struct bpf_reg_state *ptr_reg,
 {
        bool mask_to_left = (opcode == BPF_ADD &&  off_is_neg) ||
                            (opcode == BPF_SUB && !off_is_neg);
-       u32 off;
+       u32 off, max;
 
        switch (ptr_reg->type) {
        case PTR_TO_STACK:
+               /* Offset 0 is out-of-bounds, but acceptable start for the
+                * left direction, see BPF_REG_FP.
+                */
+               max = MAX_BPF_STACK + mask_to_left;
                /* Indirect variable offset stack access is prohibited in
                 * unprivileged mode so it's not handled here.
                 */
@@ -5401,15 +5405,16 @@ static int retrieve_ptr_limit(const struct bpf_reg_state *ptr_reg,
                        *ptr_limit = MAX_BPF_STACK + off;
                else
                        *ptr_limit = -off - 1;
-               return 0;
+               return *ptr_limit >= max ? -ERANGE : 0;
        case PTR_TO_MAP_VALUE:
+               max = ptr_reg->map_ptr->value_size;
                if (mask_to_left) {
                        *ptr_limit = ptr_reg->umax_value + ptr_reg->off;
                } else {
                        off = ptr_reg->smin_value + ptr_reg->off;
                        *ptr_limit = ptr_reg->map_ptr->value_size - off - 1;
                }
-               return 0;
+               return *ptr_limit >= max ? -ERANGE : 0;
        default:
                return -EINVAL;
        }