[SRU Bionic 1/1] bpf: Fix mask direction swap upon off reg sign change

[Thadeu Lima de Souza Cascardo]

From: Daniel Borkmann <daniel at iogearbox.net>

Masking direction as indicated via mask_to_left is considered to be
calculated once and then used to derive pointer limits. Thus, this
needs to be placed into bpf_sanitize_info instead so we can pass it
to sanitize_ptr_alu() call after the pointer move. Piotr noticed a
corner case where the off reg causes masking direction change which
then results in an incorrect final aux->alu_limit.

Fixes: 7fedb63a8307 ("bpf: Tighten speculative pointer arithmetic mask")
Reported-by: Piotr Krysiuk <piotras at gmail.com>
Signed-off-by: Daniel Borkmann <daniel at iogearbox.net>
Reviewed-by: Piotr Krysiuk <piotras at gmail.com>
Acked-by: Alexei Starovoitov <ast at kernel.org>
(cherry picked from commit bb01a1bba579b4b1c5566af24d95f1767859771e)
CVE-2021-33200
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo at canonical.com>
---
 kernel/bpf/verifier.c | 22 ++++++++++++----------
 1 file changed, 12 insertions(+), 10 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 60952535b626..daaec44afd04 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -1992,18 +1992,10 @@ enum {
 };
 
 static int retrieve_ptr_limit(const struct bpf_reg_state *ptr_reg,
-			      const struct bpf_reg_state *off_reg,
-			      u32 *alu_limit, u8 opcode)
+			      u32 *alu_limit, bool mask_to_left)
 {
-	bool off_is_neg = off_reg->smin_value < 0;
-	bool mask_to_left = (opcode == BPF_ADD &&  off_is_neg) ||
-			    (opcode == BPF_SUB && !off_is_neg);
 	u32 max = 0, ptr_limit = 0;
 
-	if (!tnum_is_const(off_reg->var_off) &&
-	    (off_reg->smin_value < 0) != (off_reg->smax_value < 0))
-		return REASON_BOUNDS;
-
 	switch (ptr_reg->type) {
 	case PTR_TO_STACK:
 		/* Offset 0 is out-of-bounds, but acceptable start for the
@@ -2071,6 +2063,7 @@ static bool sanitize_needed(u8 opcode)
 
 struct bpf_sanitize_info {
 	struct bpf_insn_aux_data aux;
+	bool mask_to_left;
 };
 
 static int sanitize_ptr_alu(struct bpf_verifier_env *env,
@@ -2102,7 +2095,16 @@ static int sanitize_ptr_alu(struct bpf_verifier_env *env,
 	if (vstate->speculative)
 		goto do_sim;
 
-	err = retrieve_ptr_limit(ptr_reg, off_reg, &alu_limit, opcode);
+	if (!commit_window) {
+		if (!tnum_is_const(off_reg->var_off) &&
+		    (off_reg->smin_value < 0) != (off_reg->smax_value < 0))
+			return REASON_BOUNDS;
+
+		info->mask_to_left = (opcode == BPF_ADD &&  off_is_neg) ||
+				     (opcode == BPF_SUB && !off_is_neg);
+	}
+
+	err = retrieve_ptr_limit(ptr_reg, &alu_limit, info->mask_to_left);
 	if (err < 0)
 		return err;
 
-- 
2.30.2