bpf: Fix precision tracking for BPF_ALU | BPF_TO_BE | BPF_END

commit 291d044fd51f8484066300ee42afecf8c8db7b3a upstream.

BPF_END and BPF_NEG has a different specification for the source bit in
the opcode compared to other ALU/ALU64 instructions, and is either
reserved or use to specify the byte swap endianness. In both cases the
source bit does not encode source operand location, and src_reg is a
reserved field.

backtrack_insn() currently does not differentiate BPF_END and BPF_NEG
from other ALU/ALU64 instructions, which leads to r0 being incorrectly
marked as precise when processing BPF_ALU | BPF_TO_BE | BPF_END
instructions. This commit teaches backtrack_insn() to correctly mark
precision for such case.

While precise tracking of BPF_NEG and other BPF_END instructions are
correct and does not need fixing, this commit opt to process all BPF_NEG
and BPF_END instructions within the same if-clause to better align with
current convention used in the verifier (e.g. check_alu_op).

Fixes: b5dc0163d8fd ("bpf: precise scalar_value tracking")
Cc: stable@vger.kernel.org
Reported-by: Mohamed Mahmoud <mmahmoud@redhat.com>
Closes: https://lore.kernel.org/r/87jzrrwptf.fsf@toke.dk
Tested-by: Toke Høiland-Jørgensen <toke@redhat.com>
Tested-by: Tao Lyu <tao.lyu@epfl.ch>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Shung-Hsi Yu <shung-hsi.yu@suse.com>
Link: https://lore.kernel.org/r/20231102053913.12004-2-shung-hsi.yu@suse.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 1897506..12d360d 100644 (file)
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -2596,7 +2596,12 @@ static int backtrack_insn(struct bpf_verifier_env *env, int idx,
        if (class == BPF_ALU || class == BPF_ALU64) {
                if (!(*reg_mask & dreg))
                        return 0;
-               if (opcode == BPF_MOV) {
+               if (opcode == BPF_END || opcode == BPF_NEG) {
+                       /* sreg is reserved and unused
+                        * dreg still need precision before this insn
+                        */
+                       return 0;
+               } else if (opcode == BPF_MOV) {
                        if (BPF_SRC(insn->code) == BPF_X) {
                                /* dreg = sreg
                                 * dreg needs precision after this insn