Patch "bpf: Fix may_goto with negative offset." has been added to the 6.9-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    bpf: Fix may_goto with negative offset.

to the 6.9-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     bpf-fix-may_goto-with-negative-offset.patch
and it can be found in the queue-6.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 87af6ba0076d14429c70e4507b9939497852242e
Author: Alexei Starovoitov <ast@xxxxxxxxxx>
Date:   Wed Jun 19 16:53:54 2024 -0700

    bpf: Fix may_goto with negative offset.
    
    [ Upstream commit 2b2efe1937ca9f8815884bd4dcd5b32733025103 ]
    
    Zac's syzbot crafted a bpf prog that exposed two bugs in may_goto.
    The 1st bug is the way may_goto is patched. When offset is negative
    it should be patched differently.
    The 2nd bug is in the verifier:
    when current state may_goto_depth is equal to visited state may_goto_depth
    it means there is an actual infinite loop. It's not correct to prune
    exploration of the program at this point.
    Note, that this check doesn't limit the program to only one may_goto insn,
    since 2nd and any further may_goto will increment may_goto_depth only
    in the queued state pushed for future exploration. The current state
    will have may_goto_depth == 0 regardless of number of may_goto insns
    and the verifier has to explore the program until bpf_exit.
    
    Fixes: 011832b97b31 ("bpf: Introduce may_goto instruction")
    Reported-by: Zac Ecob <zacecob@xxxxxxxxxxxxxx>
    Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxx>
    Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
    Signed-off-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
    Acked-by: Eduard Zingerman <eddyz87@xxxxxxxxx>
    Closes: https://lore.kernel.org/bpf/CAADnVQL-15aNp04-cyHRn47Yv61NXfYyhopyZtUyxNojUZUXpA@xxxxxxxxxxxxxx/
    Link: https://lore.kernel.org/bpf/20240619235355.85031-1-alexei.starovoitov@xxxxxxxxx
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 2233bf50a9012..ab558eea1c9ee 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -17308,11 +17308,11 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx)
 				goto skip_inf_loop_check;
 			}
 			if (is_may_goto_insn_at(env, insn_idx)) {
-				if (states_equal(env, &sl->state, cur, RANGE_WITHIN)) {
+				if (sl->state.may_goto_depth != cur->may_goto_depth &&
+				    states_equal(env, &sl->state, cur, RANGE_WITHIN)) {
 					update_loop_entry(cur, &sl->state);
 					goto hit;
 				}
-				goto skip_inf_loop_check;
 			}
 			if (calls_callback(env, insn_idx)) {
 				if (states_equal(env, &sl->state, cur, RANGE_WITHIN))
@@ -19853,7 +19853,10 @@ static int do_misc_fixups(struct bpf_verifier_env *env)
 
 			stack_depth_extra = 8;
 			insn_buf[0] = BPF_LDX_MEM(BPF_DW, BPF_REG_AX, BPF_REG_10, stack_off);
-			insn_buf[1] = BPF_JMP_IMM(BPF_JEQ, BPF_REG_AX, 0, insn->off + 2);
+			if (insn->off >= 0)
+				insn_buf[1] = BPF_JMP_IMM(BPF_JEQ, BPF_REG_AX, 0, insn->off + 2);
+			else
+				insn_buf[1] = BPF_JMP_IMM(BPF_JEQ, BPF_REG_AX, 0, insn->off - 1);
 			insn_buf[2] = BPF_ALU64_IMM(BPF_SUB, BPF_REG_AX, 1);
 			insn_buf[3] = BPF_STX_MEM(BPF_DW, BPF_REG_10, BPF_REG_AX, stack_off);
 			cnt = 4;