[PATCH bpf-next v2 1/2] bpf, verifier: fold reg->var_off into PTR_TO_FLOW_KEYS bounds check

[Nuoqi Gui]

Constant pointer arithmetic on a PTR_TO_FLOW_KEYS register lands the
constant in reg->var_off (e.g. flow_keys(imm=4096)), but the
PTR_TO_FLOW_KEYS path in check_mem_access() passes only insn->off to
check_flow_keys_access() and never folds reg->var_off.value.  The
verifier therefore accepts an access that, at runtime, dereferences past
struct bpf_flow_keys -- a verifier/runtime divergence that yields an
out-of-bounds read and write of kernel stack memory.

Commit 022ac0750883 ("bpf: use reg->var_off instead of reg->off for
pointers") removed the generic "off += reg->off" that check_mem_access()
applied before the per-type dispatch and replaced it with per-path
folding of reg->var_off.value (for example the ctx path now folds the
register offset via check_ctx_access()).  The PTR_TO_FLOW_KEYS path was
not given the equivalent fold, so a constant offset that used to be
folded and rejected is now silently accepted:

  before 022ac0750883: the offset stays in reg->off and is folded
    generically, so the access is checked with off=4096 and rejected.
  after  022ac0750883: the offset lands in reg->var_off, the flow_keys
    path checks off=0 and accepts; at runtime the access dereferences
    base + 0x1000.

For a BPF_PROG_TYPE_FLOW_DISSECTOR program the following is accepted:

  r2 = *(u64 *)(r1 + 144)   ; R2=flow_keys (PTR_TO_FLOW_KEYS)
  r2 += 0x1000              ; R2=flow_keys(imm=4096), accepted
  r0 = *(u64 *)(r2 + 0)     ; accepted, var_off.value=0x1000 ignored

while the equivalent insn->off form

  r0 = *(u64 *)(r2 + 0x1000)

has the same effective offset but is correctly rejected with
"invalid access to flow keys off=4096 size=8", which isolates the defect
to the missing var_off fold.  Once attached as a flow dissector, the
accepted program reads kernel stack past struct bpf_flow_keys (a
kernel-stack / KASLR information leak) and can likewise write past it,
corrupting kernel memory.

Fix it by folding reg->var_off.value into the offset before the bounds
check and rejecting non-constant offsets, mirroring the other pointer
types (e.g. check_ctx_access()).

No released kernel is affected; the regression is confined to the 7.1
development cycle (reproduced on v7.1-rc1..rc5), and v7.0.x rejects the
program above.

Fixes: 022ac0750883 ("bpf: use reg->var_off instead of reg->off for pointers")
Signed-off-by: Nuoqi Gui <gnq25@xxxxxxxxxxxxxxxxxxxxx>
---
 kernel/bpf/verifier.c | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 8ed484cb1a8a..c04941636ef4 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -4728,9 +4728,22 @@ static int check_ctx_access(struct bpf_verifier_env *env, int insn_idx, struct b
 	return err;
 }
 
-static int check_flow_keys_access(struct bpf_verifier_env *env, int off,
-				  int size)
+static int check_flow_keys_access(struct bpf_verifier_env *env, u32 regno,
+				  int off, int size)
 {
+	struct bpf_reg_state *reg = reg_state(env, regno);
+
+	/* Only a constant offset is allowed here; fold it into off. */
+	if (!tnum_is_const(reg->var_off)) {
+		char tn_buf[48];
+
+		tnum_strn(tn_buf, sizeof(tn_buf), reg->var_off);
+		verbose(env, "R%d invalid variable offset to flow keys: off=%d, var_off=%s\n",
+			regno, off, tn_buf);
+		return -EACCES;
+	}
+	off += reg->var_off.value;
+
 	if (size < 0 || off < 0 ||
 	    (u64)off + size > sizeof(struct bpf_flow_keys)) {
 		verbose(env, "invalid access to flow keys off=%d size=%d\n",
@@ -6239,7 +6252,7 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, struct b
 			return -EACCES;
 		}
 
-		err = check_flow_keys_access(env, off, size);
+		err = check_flow_keys_access(env, regno, off, size);
 		if (!err && t == BPF_READ && value_regno >= 0)
 			mark_reg_unknown(env, regs, value_regno);
 	} else if (type_is_sk_pointer(reg->type)) {
-- 
2.34.1