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[83.42.57.116]) by smtp.gmail.com with ESMTPSA id l16sm2132781edv.71.2020.04.15.06.02.18 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 15 Apr 2020 06:02:20 -0700 (PDT) Subject: Re: [PATCH-for-5.0 04/12] qga: Restrict guest-file-read count to 48 MB to avoid crashes To: =?UTF-8?Q?Daniel_P=2e_Berrang=c3=a9?= References: <20200414133052.13712-1-philmd@redhat.com> <20200414133052.13712-5-philmd@redhat.com> <20200415123405.GD1344391@redhat.com> From: =?UTF-8?Q?Philippe_Mathieu-Daud=c3=a9?= Message-ID: <3cd57dd9-0552-8064-c9d8-43913ed73e2e@redhat.com> Date: Wed, 15 Apr 2020 15:02:18 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.5.0 MIME-Version: 1.0 In-Reply-To: <20200415123405.GD1344391@redhat.com> Content-Language: en-US X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: quoted-printable X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 205.139.110.120 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: Kevin Wolf , Peter Maydell , Michael Roth , qemu-block@nongnu.org, "Michael S. Tsirkin" , Fakhri Zulkifli , qemu-devel@nongnu.org, Fabien Chouteau , Max Filippov , KONRAD Frederic , qemu-arm@nongnu.org, qemu-ppc@nongnu.org, Gerd Hoffmann , =?UTF-8?Q?Marc-Andr=c3=a9_Lureau?= , Stafford Horne , Max Reitz , Aurelien Jarno Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 4/15/20 2:34 PM, Daniel P. Berrang=C3=A9 wrote: > On Tue, Apr 14, 2020 at 03:30:44PM +0200, Philippe Mathieu-Daud=C3=A9 wro= te: >> On [*] Daniel Berrang=C3=A9 commented: >> >> The QEMU guest agent protocol is not sensible way to access huge >> files inside the guest. It requires the inefficient process of >> reading the entire data into memory than duplicating it again in >> base64 format, and then copying it again in the JSON serializer / >> monitor code. >> >> For arbitrary general purpose file access, especially for large >> files, use a real file transfer program or use a network block >> device, not the QEMU guest agent. >> >> To avoid bug reports as BZ#1594054 (CVE-2018-12617), follow his >> suggestion to put a low, hard limit on "count" in the guest agent >> QAPI schema, and don't allow count to be larger than 48 MB. >> >> [*] https://www.mail-archive.com/qemu-devel@nongnu.org/msg693176.html >> >> Fixes: CVE-2018-12617 >> Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=3D1594054 >> Reported-by: Fakhri Zulkifli >> Suggested-by: Daniel P. Berrang=C3=A9 >> Signed-off-by: Philippe Mathieu-Daud=C3=A9 >> --- >> qga/qapi-schema.json | 6 ++++-- >> qga/commands.c | 9 ++++++++- >> 2 files changed, 12 insertions(+), 3 deletions(-) >> >> diff --git a/qga/qapi-schema.json b/qga/qapi-schema.json >> index f6fcb59f34..7758d9daf8 100644 >> --- a/qga/qapi-schema.json >> +++ b/qga/qapi-schema.json >> @@ -266,11 +266,13 @@ >> ## >> # @guest-file-read: >> # >> -# Read from an open file in the guest. Data will be base64-encoded >> +# Read from an open file in the guest. Data will be base64-encoded. >> +# As this command is just for limited, ad-hoc debugging, such as log >> +# file access, the number of bytes to read is limited to 10 MB. >=20 > s/10/48/ >=20 >> # >> # @handle: filehandle returned by guest-file-open >> # >> -# @count: maximum number of bytes to read (default is 4KB) >> +# @count: maximum number of bytes to read (default is 4KB, maximum is 1= 0MB) >=20 > s/10/48/ Oops I totally missed those, thanks! >=20 >> # >> # Returns: @GuestFileRead on success. >> # >> diff --git a/qga/commands.c b/qga/commands.c >> index 5611117372..efc8b90281 100644 >> --- a/qga/commands.c >> +++ b/qga/commands.c >> @@ -11,6 +11,7 @@ >> */ >> =20 >> #include "qemu/osdep.h" >> +#include "qemu/units.h" >> #include "guest-agent-core.h" >> #include "qga-qapi-commands.h" >> #include "qapi/error.h" >> @@ -24,6 +25,12 @@ >> #define GUEST_EXEC_MAX_OUTPUT (16*1024*1024) >> /* Allocation and I/O buffer for reading guest-exec out_data/err_data = - 4KB */ >> #define GUEST_EXEC_IO_SIZE (4*1024) >> +/* >> + * Maximum file size to read - 48MB >> + * >> + * (48MB + Base64 3:4 overhead =3D JSON parser 64 MB limit) >> + */ >> +#define GUEST_FILE_READ_COUNT_MAX (48 * MiB) >> =20 >> /* Note: in some situations, like with the fsfreeze, logging may be >> * temporarilly disabled. if it is necessary that a command be able >> @@ -560,7 +567,7 @@ GuestFileRead *qmp_guest_file_read(int64_t handle, b= ool has_count, >> } >> if (!has_count) { >> count =3D QGA_READ_COUNT_DEFAULT; >> - } else if (count < 0 || count >=3D UINT32_MAX) { >> + } else if (count < 0 || count > GUEST_FILE_READ_COUNT_MAX) { >> error_setg(errp, "value '%" PRId64 "' is invalid for argument = count", >> count); >> return NULL; >=20 > With the docs typos fixed: >=20 > Reviewed-by: Daniel P. Berrang=C3=A9 >=20 >=20 > Regards, > Daniel >=20