From nobody Fri Sep 25 09:22:26 2026 Received: from out28-123.mail.aliyun.com (out28-123.mail.aliyun.com [115.124.28.123]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id BFD96486433; Mon, 14 Sep 2026 18:04:05 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=115.124.28.123 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789409049; cv=none; b=CwjtfRH3ur2hJcfx6Bb4EeCOVoHdWkROlotte/03gqNvl+3uhtEEFpyRzs9STqqkknOEQFDqansJ3JJ557+mAsvgYeUY9vNenLNIZhraDJ8vVnVP7Ey+xoYtryOpMTG8PpgywBJxFH2ymSyX5OsjpJTGzcgNu9RB4KsaJoNSk7E= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789409049; c=relaxed/simple; bh=ZUnyJvSfvQf/+1qB1TzF9Mo2CZ07AJi/g0lVpvGkQ2c=; h=From:To:Cc:Subject:Date:Message-ID:MIME-Version; b=dwlc6OiIQpwWCTvSA6ra2nHyv4jxUMRTDREjMPmFVICpB8ar57fXPcr+B5jtEWFcXvy4uWWq2uu6tmaob2jW+VdrPjQueXsSUAOjnnVfI4uYZLtFSqbJUhHVenwP78rrU/q/+xc+vCgrU2CmxLsAs2Zpkw7uNdsWnfb8zKEyjhw= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=xiaopeng.com; spf=pass smtp.mailfrom=xiaopeng.com; dkim=pass (1024-bit key) header.d=xiaopeng.com header.i=@xiaopeng.com header.b=GLx9D2Gd; arc=none smtp.client-ip=115.124.28.123 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=xiaopeng.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=xiaopeng.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=xiaopeng.com header.i=@xiaopeng.com header.b="GLx9D2Gd" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=xiaopeng.com; s=default; t=1789409043; h=From:To:Subject:Date:Message-ID:MIME-Version; bh=KoqZItCTin+6stOEbAOp7G8XLkIXJ9u/snZJG1JONh0=; b=GLx9D2Gd+80Sf2zdV5htgldO3yDOcZYfJxp3bfxk7c6ZJu+mzbQ/py6Q9Q0/c01fyGaD91zxahqBb3YhKBmZhotlDMhDcsCZno/yLRH4UWLlWCiIyB2Q+8W9yjEQ6CudisWhnvOKh5t1wMDGKnYew6XEaSG0T2bHTcXJ+OVtOi8= X-Alimail-AntiSpam: AC=CONTINUE;BC=0.04477426|-1;CH=green;DM=|CONTINUE|false|;DS=CONTINUE|ham_alarm|0.0082244-0.000528985-0.991247;FP=10884422329824779518|0|0|0|0|-1|-1|-1;HT=maildocker-contentspam033037032089;MF=fangxy@xiaopeng.com;NM=1;PH=DS;RN=6;RT=6;SR=0;TI=SMTPD_---.jDU33YF_1789409024; Received: from localhost.localdomain(mailfrom:fangxy@xiaopeng.com fp:SMTPD_---.jDU33YF_1789409024 cluster:ay29) by smtp.aliyun-inc.com; Tue, 15 Sep 2026 02:04:02 +0800 From: Fang Xieyan To: jason.wessel@windriver.com, danielt@kernel.org, dianders@chromium.org Cc: kgdb-bugreport@lists.sourceforge.net, linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: [PATCH] kgdb: Fix buffer overflow in the 'm' packet handler Date: Tue, 15 Sep 2026 02:03:44 +0800 Message-ID: <20260914180344.15485-1-fangxy@xiaopeng.com> X-Mailer: git-send-email 2.50.1 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" gdb_cmd_memread() passes the length of the 'm' packet straight to kgdb_mem2hex(), which hex-encodes the reply into remcom_out_buffer[BUFMAX] without checking that the reply fits. A client on the KGDB I/O console can ask for a read of any size, and the hex encoding runs off the end of the buffer. kgdb_mem2hex() takes that length as an int count, so 2^31 truncates to a negative value and copy_from_kernel_nofault() is handed buf + count as its destination. On x86 BUFMAX is 1024, so a read of 512 bytes overruns by the single byte that terminates the hex string: BUG: KASAN: global-out-of-bounds in kgdb_mem2hex+0x1dd/0x230 Write of size 1 at addr ffffffff87651780 by task sh/1 ... The buggy address belongs to the variable: remcom_out_buffer+0x400/0x420 Bounds check the length before the hex encoding: if (length > (BUFMAX - 1) / 2) { error_packet(remcom_out_buffer, -EINVAL); return; } gdbstub_msg_write() already bounds the count it hex-encodes against BUFMAX. Reads over the limit never returned valid data, so an error reply loses nothing. Fixes: dc7d55270521 ("kgdb: core") Cc: stable@vger.kernel.org Assisted-by: Hawkeye:GLM-5.3-flash Assisted-by: Qoder:Qwen3.8-Max Signed-off-by: Fang Xieyan Reviewed-by: Douglas Anderson --- The scanner is a taint-style review of the packet handlers in kernel/debug/gdbstub.c. write_mem_msg() ('M' and 'X' packets) trusts the same length field and looks like the same class of problem; I can send that separately if there is interest. Reproducer: attach to a kgdboc console and send an 'm' packet with a length above (BUFMAX - 1) / 2, e.g. "m,200" on x86. Before this change the hex encoding runs off the end of remcom_out_buffer; after it the stub answers E22 and the guest resumes. Both cases were run on 704340f1cd0d (v7.3-rc4): x86_64 defconfig plus CONFIG_KASAN_GENERIC, gcc 13.2.0, QEMU under TCG. The two kernels are built from byte-identical .config files and differ only by this patch. Two details of the setup are not obvious. CONFIG_KGDB_KDB has to be off: dbg_kdb_mode starts at 1, so with kdb built in a sysrq-g break lands in kdb_stub() and the packet is never parsed. A minimal over-the-limit length reproduces better than a large one. copy_from_kernel_nofault() copies through raw asm accessors that KASAN does not instrument, so length =3D 0x10000 writes 64 KiB past the buffer with no report at all and the kernel simply stops answering. length =3D 0x200 keeps that copy inside the buffer, which leaves the terminating store as the only out-of-bounds access and gives the report above. kernel/debug/gdbstub.c | 8 ++++++++ 1 file changed, 8 insertions(+) diff --git a/kernel/debug/gdbstub.c b/kernel/debug/gdbstub.c index e271a43..5774292 100644 --- a/kernel/debug/gdbstub.c +++ b/kernel/debug/gdbstub.c @@ -563,6 +563,14 @@ static void gdb_cmd_memread(struct kgdb_state *ks) =20 if (kgdb_hex2long(&ptr, &addr) > 0 && *ptr++ =3D=3D ',' && kgdb_hex2long(&ptr, &length) > 0) { + /* + * The hex-encoded reply needs 2 * length + 1 bytes + * in remcom_out_buffer[BUFMAX]. + */ + if (length > (BUFMAX - 1) / 2) { + error_packet(remcom_out_buffer, -EINVAL); + return; + } err =3D kgdb_mem2hex((char *)addr, remcom_out_buffer, length); if (!err) error_packet(remcom_out_buffer, -EINVAL); --=20 2.50.1 (Apple Git-155)