From: Damien Hedde <damien.hedde@greensocs.com>
To: "Alex Bennée" <alex.bennee@linaro.org>
Cc: philmd@redhat.com, qemu-devel@nongnu.org
Subject: Re: [PATCH] gdbstub: Fix buffer overflow in handle_read_all_regs
Date: Fri, 8 Nov 2019 15:52:23 +0100 [thread overview]
Message-ID: <2be19dae-bc42-972c-18d9-ab332e98c825@greensocs.com> (raw)
In-Reply-To: <7aa732a4-b67f-855f-0432-290580fc239d@greensocs.com>
On 11/8/19 3:43 PM, Damien Hedde wrote:
>
>
> On 11/8/19 3:09 PM, Alex Bennée wrote:
>>
>> Damien Hedde <damien.hedde@greensocs.com> 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 <damien.hedde@greensocs.com>
>>> ---
>>>
>>> Hi all,
>>>
>>> While working on a target with many registers. I found out the gdbstub
>>> 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 get
>> quite big).
>
> SVE ?
>
>>
>>>
>>> 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(GdbCmdContext *gdb_ctx, void *user_ctx)
>>> cpu_synchronize_state(gdb_ctx->s->g_cpu);
>>> len = 0;
>>> for (addr = 0; addr < gdb_ctx->s->g_cpu->gdb_num_g_regs; addr++) {
>>> - len += gdb_read_register(gdb_ctx->s->g_cpu, gdb_ctx->mem_buf + len,
>>> - addr);
>>> + int size = 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() below.
>>> + * As a consequence, send only the first registers content.
>>> + * 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)
sorry, it is MAX_PACKET_LENGTH / 2 for the max single register allowed size.
> str_buf has a size of MAX_PACKET_LENGTH + 1
>
> I'm not sure I've understood the second part but if we increase the size
> of str_buf then we will need also a bigger packet buffer.
>
> The size here only depends on what are the target declared registers, so
> it depends only on the cpu target code.
>
>>
>>> + break;
>>> + }
>>> + len += size;
>>> }
>>>
>>> memtohex(gdb_ctx->str_buf, gdb_ctx->mem_buf, len);
>>
>>
>> --
>> Alex Bennée
>>
>
next prev parent reply other threads:[~2019-11-08 14:55 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-11-08 12:55 [PATCH] gdbstub: Fix buffer overflow in handle_read_all_regs Damien Hedde
2019-11-08 13:40 ` Luc Michel
2019-11-08 14:09 ` Alex Bennée
2019-11-08 14:43 ` Damien Hedde
2019-11-08 14:52 ` Damien Hedde [this message]
2019-11-08 16:50 ` Alex Bennée
2019-11-14 10:19 ` Damien Hedde
2019-11-14 13:47 ` Alex Bennée
2019-11-14 14:43 ` Damien Hedde
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=2be19dae-bc42-972c-18d9-ab332e98c825@greensocs.com \
--to=damien.hedde@greensocs.com \
--cc=alex.bennee@linaro.org \
--cc=philmd@redhat.com \
--cc=qemu-devel@nongnu.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).