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Thu, 14 Nov 2019 10:19:18 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=greensocs.com; s=mail; t=1573726758; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=63vYYUWDpprINMCK0qNnk+1Ba0UCWFiZtg9mSDtgwiI=; b=3178B9MgLp37OhpB7fOBTSv1OZLOcr4Emk6R1QmVdiUGO3uMcU/hDkm2UvXSL7VNJtUxRE zEoCKiQl3R0HwGQ9HIhvctnkgx2ceTVZr6T7Wf6xTThlm8Ml0yW6dETZOPcvTKUBrvjTjF vd1gW/CRNpfe+tple1me2wZhFUXd0i4= Subject: Re: [PATCH] gdbstub: Fix buffer overflow in handle_read_all_regs To: =?UTF-8?Q?Alex_Benn=c3=a9e?= References: <20191108125534.114474-1-damien.hedde@greensocs.com> <877e4ah32n.fsf@linaro.org> <7aa732a4-b67f-855f-0432-290580fc239d@greensocs.com> <87v9rufh2z.fsf@linaro.org> From: Damien Hedde Message-ID: <78291aad-5c91-32a3-f0d8-f74f3a75dfbf@greensocs.com> Date: Thu, 14 Nov 2019 11:19:17 +0100 User-Agent: Mozilla/5.0 (X11; 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ORIGINATING; auth=pass smtp.auth=damien smtp.mailfrom=damien.hedde@greensocs.com Content-Transfer-Encoding: quoted-printable X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 5.135.226.135 X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: philmd@redhat.com, qemu-devel@nongnu.org, Luc Michel Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 11/8/19 5:50 PM, Alex Benn=C3=A9e wrote: >=20 > Damien Hedde writes: >=20 >> On 11/8/19 3:09 PM, Alex Benn=C3=A9e wrote: >>> >>> Damien Hedde writes: >>> >>>> Ensure we don't put too much register data in buffers. This avoids >>>> a buffer overflow (and stack corruption) when a target has lots >>>> of registers. >>>> >>>> Signed-off-by: Damien Hedde >>>> --- >>>> >>>> Hi all, >>>> >>>> While working on a target with many registers. I found out the gdbst= ub >>>> may do buffer overflows when receiving a 'g' query (to read general >>>> registers). This patch prevents that. >>>> >>>> Gdb is pretty happy with a partial set of registers and queries >>>> remaining registers one by one when needed. >>> >>> Heh I was just looking at this code with regards to SVE (which can ge= t >>> quite big). >> >> SVE ? >=20 > ARM's Scalable Vector Registers which currently can get upto 16 vector > quads (256 bytes) but are likely to get bigger. >=20 >> >>> >>>> >>>> Regards, >>>> Damien >>>> --- >>>> gdbstub.c | 13 +++++++++++-- >>>> 1 file changed, 11 insertions(+), 2 deletions(-) >>>> >>>> diff --git a/gdbstub.c b/gdbstub.c >>>> index 4cf8af365e..dde0cfe0fe 100644 >>>> --- a/gdbstub.c >>>> +++ b/gdbstub.c >>>> @@ -1810,8 +1810,17 @@ static void handle_read_all_regs(GdbCmdContex= t *gdb_ctx, void *user_ctx) >>>> cpu_synchronize_state(gdb_ctx->s->g_cpu); >>>> len =3D 0; >>>> for (addr =3D 0; addr < gdb_ctx->s->g_cpu->gdb_num_g_regs; addr= ++) { >>>> - len +=3D gdb_read_register(gdb_ctx->s->g_cpu, gdb_ctx->mem_= buf + len, >>>> - addr); >>>> + int size =3D gdb_read_register(gdb_ctx->s->g_cpu, gdb_ctx->= mem_buf + len, >>>> + addr); >>>> + if (len + size > MAX_PACKET_LENGTH / 2) { >>>> + /* >>>> + * Prevent gdb_ctx->str_buf overflow in memtohex() belo= w. >>>> + * As a consequence, send only the first registers cont= ent. >>>> + * Gdb will query remaining ones if/when needed. >>>> + */ >>> >>> Haven't we already potentially overflowed gdb_ctx->mem_buf though? I >>> suspect the better fix is for str_buf is to make it growable with >>> g_string and be able to handle arbitrary size conversions (unless the >>> spec limits us). But we still don't want a hostile gdbstub to be able= to >>> spam memory by asking for registers that might be bigger than >>> MAX_PACKET_LENGTH bytes. >> >> For gdb_ctx->mem_buf it's ok because it has also a size of >> MAX_PACKET_LENGTH. (assuming no single register can be bigger than >> MAX_PACKET_LENGTH) >> str_buf has a size of MAX_PACKET_LENGTH + 1 >=20 > Are these limits of the protocol rather than our own internal limits? gdb has a dynamic sized packet buffer. Remote protocol doc says: =E2=80=98qSupported [:gdbfeature [;gdbfeature]=E2=80=A6 ]=E2=80=99 [...] Any GDB which sends a =E2=80=98qSupported=E2=80=99 packet suppo= rts receiving packets of unlimited length (earlier versions of GDB may reject overly long responses). >=20 >> I'm not sure I've understood the second part but if we increase the si= ze >> of str_buf then we will need also a bigger packet buffer. >=20 > Glib provides some nice functions for managing arbitrary sized strings > in a nice flexible way which grow on demand. There is also a nice > growable GByteArray type which we can use for the packet buffer. I thin= k > I'd started down this road of re-factoring but never got around to > posting the patches. >=20 >> The size here only depends on what are the target declared registers, = so >> it depends only on the cpu target code. >=20 > Sure - but guest registers are growing all the time! >=20 > -- > Alex Benn=C3=A9e >=20