linux-kernel - [PATCH 5.10 205/563] bpf: Dont promote bogus looking registers after null check.

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Date:   Mon, 24 Jan 2022 19:39:30 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH 5.10 205/563] bpf: Dont promote bogus looking registers after null check.

From: Daniel Borkmann <daniel@...earbox.net>

[ Upstream commit e60b0d12a95dcf16a63225cead4541567f5cb517 ]

If we ever get to a point again where we convert a bogus looking <ptr>_or_null
typed register containing a non-zero fixed or variable offset, then lets not
reset these bounds to zero since they are not and also don't promote the register
to a <ptr> type, but instead leave it as <ptr>_or_null. Converting to a unknown
register could be an avenue as well, but then if we run into this case it would
allow to leak a kernel pointer this way.

Fixes: f1174f77b50c ("bpf/verifier: rework value tracking")
Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
Signed-off-by: Alexei Starovoitov <ast@...nel.org>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 kernel/bpf/verifier.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index c623c3e549210..015bf2ba4a0b6 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -7725,15 +7725,15 @@ static void mark_ptr_or_null_reg(struct bpf_func_state *state,
 {
 	if (reg_type_may_be_null(reg->type) && reg->id == id &&
 	    !WARN_ON_ONCE(!reg->id)) {
-		/* Old offset (both fixed and variable parts) should
-		 * have been known-zero, because we don't allow pointer
-		 * arithmetic on pointers that might be NULL.
-		 */
 		if (WARN_ON_ONCE(reg->smin_value || reg->smax_value ||
 				 !tnum_equals_const(reg->var_off, 0) ||
 				 reg->off)) {
-			__mark_reg_known_zero(reg);
-			reg->off = 0;
+			/* Old offset (both fixed and variable parts) should
+			 * have been known-zero, because we don't allow pointer
+			 * arithmetic on pointers that might be NULL. If we
+			 * see this happening, don't convert the register.
+			 */
+			return;
 		}
 		if (is_null) {
 			reg->type = SCALAR_VALUE;
-- 
2.34.1