[PATCH 6.1 168/372] bpf: Fix precision tracking for BPF_ALU | BPF_TO_BE | BPF_END

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6.1-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Shung-Hsi Yu <shung-hsi.yu@xxxxxxxx>

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@xxxxxxxxxxxxxxx
Reported-by: Mohamed Mahmoud <mmahmoud@xxxxxxxxxx>
Closes: https://lore.kernel.org/r/87jzrrwptf.fsf@xxxxxxx
Tested-by: Toke Høiland-Jørgensen <toke@xxxxxxxxxx>
Tested-by: Tao Lyu <tao.lyu@xxxxxxx>
Acked-by: Eduard Zingerman <eddyz87@xxxxxxxxx>
Signed-off-by: Shung-Hsi Yu <shung-hsi.yu@xxxxxxxx>
Link: https://lore.kernel.org/r/20231102053913.12004-2-shung-hsi.yu@xxxxxxxx
Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 kernel/bpf/verifier.c |    7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -2596,7 +2596,12 @@ static int backtrack_insn(struct bpf_ver
 	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






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