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[83.42.57.116]) by smtp.gmail.com with ESMTPSA id 9sm1557529ejv.54.2020.03.31.07.15.41 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 31 Mar 2020 07:15:42 -0700 (PDT) Subject: Re: [PATCH-for-5.0 v2 1/4] Revert "prevent crash when executing guest-file-read with large count" To: =?UTF-8?Q?Daniel_P=2e_Berrang=c3=a9?= References: <20200331140638.16464-1-philmd@redhat.com> <20200331140638.16464-2-philmd@redhat.com> <20200331141205.GI353752@redhat.com> From: =?UTF-8?Q?Philippe_Mathieu-Daud=c3=a9?= Message-ID: Date: Tue, 31 Mar 2020 16:15:41 +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: <20200331141205.GI353752@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] [fuzzy] X-Received-From: 207.211.31.81 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: qemu-devel@nongnu.org, Michael Roth , Fakhri Zulkifli , Markus Armbruster , Sameeh Jubran , Basil Salman , Dietmar Maurer , "Dr . David Alan Gilbert" Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 3/31/20 4:12 PM, Daniel P. Berrang=C3=A9 wrote: > On Tue, Mar 31, 2020 at 04:06:35PM +0200, Philippe Mathieu-Daud=C3=A9 wro= te: >> By using g_try_malloc() instead of g_malloc() the qemu-guest-agent >> Denial-of-Service attack referred in commit 807e2b6fce is reduced, >> but still triggerable: >=20 > As explained previously, I believe there is *no* denial of service > attack here. The described scenario is just a user hurting themselves > by intentionally telling QEMU not to limit the amount of data returned. Yes. Do you mind updating the BZ, eventually marking as NOTABUG? Then I=20 can adapt the patch descriptions. https://bugzilla.redhat.com/show_bug.cgi?id=3D1594054 >=20 >> >> - bisect file size S until g_try_malloc(S) fails, >> - use S - 1: >> g_try_malloc(S - 1) succeeds, but g_new0() few lines later will >> fail. >> >> 346 buf =3D g_try_malloc0(count + 1); >> 347 if (!buf) { >> 348 error_setg(errp, >> 349 "failed to allocate sufficient memory " >> 350 "to complete the requested service"); >> 351 return NULL; >> 352 } >> 353 is_ok =3D ReadFile(fh, buf, count, &read_count, NULL); >> 354 if (!is_ok) { >> 355 error_setg_win32(errp, GetLastError(), "failed to read fil= e"); >> 356 slog("guest-file-read failed, handle %" PRId64, handle); >> 357 } else { >> 358 buf[read_count] =3D 0; >> 359 read_data =3D g_new0(GuestFileRead, 1); >> ^^^^^^ >> >> Instead we are going to put a low hard limit on 'count' in the next >> commits. >> This reverts commit 807e2b6fce022707418bc8f61c069d91c613b3d2. >> >> Suggested-by: Daniel P. Berrang=C3=A9 >> Signed-off-by: Philippe Mathieu-Daud=C3=A9 >> --- >> qga/commands-win32.c | 8 +------- >> 1 file changed, 1 insertion(+), 7 deletions(-) >> >> diff --git a/qga/commands-win32.c b/qga/commands-win32.c >> index b49920e201..46cea7d1d9 100644 >> --- a/qga/commands-win32.c >> +++ b/qga/commands-win32.c >> @@ -343,13 +343,7 @@ GuestFileRead *qmp_guest_file_read(int64_t handle, = bool has_count, >> } >> =20 >> fh =3D gfh->fh; >> - buf =3D g_try_malloc0(count + 1); >> - if (!buf) { >> - error_setg(errp, >> - "failed to allocate sufficient memory " >> - "to complete the requested service"); >> - return NULL; >> - } >> + buf =3D g_malloc0(count + 1); >> is_ok =3D ReadFile(fh, buf, count, &read_count, NULL); >> if (!is_ok) { >> error_setg_win32(errp, GetLastError(), "failed to read file"); >> --=20 >> 2.21.1 >> >=20 > Regards, > Daniel >=20