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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Piotr Krysiuk <piotras@gmail.com>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Alexei Starovoitov <ast@kernel.org>
Subject: [PATCH 5.11 23/31] bpf: Add sanity check for upper ptr_limit
Date:   Fri, 19 Mar 2021 13:19:17 +0100
Message-Id: <20210319121747.948248574@linuxfoundation.org>
In-Reply-To: <20210319121747.203523570@linuxfoundation.org>
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From: Piotr Krysiuk <piotras@gmail.com>

commit 1b1597e64e1a610c7a96710fc4717158e98a08b3 upstream.

Given we know the max possible value of ptr_limit at the time of retrieving
the latter, add basic assertions, so that the verifier can bail out if
anything looks odd and reject the program. Nothing triggered this so far,
but it also does not hurt to have these.

Signed-off-by: Piotr Krysiuk <piotras@gmail.com>
Co-developed-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 kernel/bpf/verifier.c |   11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -5389,10 +5389,14 @@ static int retrieve_ptr_limit(const stru
 {
 	bool mask_to_left = (opcode == BPF_ADD &&  off_is_neg) ||
 			    (opcode == BPF_SUB && !off_is_neg);
-	u32 off;
+	u32 off, max;
 
 	switch (ptr_reg->type) {
 	case PTR_TO_STACK:
+		/* Offset 0 is out-of-bounds, but acceptable start for the
+		 * left direction, see BPF_REG_FP.
+		 */
+		max = MAX_BPF_STACK + mask_to_left;
 		/* Indirect variable offset stack access is prohibited in
 		 * unprivileged mode so it's not handled here.
 		 */
@@ -5401,15 +5405,16 @@ static int retrieve_ptr_limit(const stru
 			*ptr_limit = MAX_BPF_STACK + off;
 		else
 			*ptr_limit = -off - 1;
-		return 0;
+		return *ptr_limit >= max ? -ERANGE : 0;
 	case PTR_TO_MAP_VALUE:
+		max = ptr_reg->map_ptr->value_size;
 		if (mask_to_left) {
 			*ptr_limit = ptr_reg->umax_value + ptr_reg->off;
 		} else {
 			off = ptr_reg->smin_value + ptr_reg->off;
 			*ptr_limit = ptr_reg->map_ptr->value_size - off - 1;
 		}
-		return 0;
+		return *ptr_limit >= max ? -ERANGE : 0;
 	default:
 		return -EINVAL;
 	}