From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:46077) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1eYtnt-0001Dx-Al for qemu-devel@nongnu.org; Tue, 09 Jan 2018 08:18:58 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1eYtnq-0005v9-75 for qemu-devel@nongnu.org; Tue, 09 Jan 2018 08:18:57 -0500 References: <20180108215007.46471-1-marcel@redhat.com> <3ea6176b-01a4-8f5a-81fc-3e9a8c846dc7@redhat.com> From: Marcel Apfelbaum Message-ID: Date: Tue, 9 Jan 2018 15:18:45 +0200 MIME-Version: 1.0 In-Reply-To: <3ea6176b-01a4-8f5a-81fc-3e9a8c846dc7@redhat.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: quoted-printable Subject: Re: [Qemu-devel] [PATCH] fw_cfg: fix memory corruption when all fw_cfg slots are used List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Laszlo Ersek , qemu-devel@nongnu.org Cc: ehabkost@redhat.com, mst@redhat.com, qemu-stable@nongnu.org, kraxel@redhat.com On 09/01/2018 15:09, Laszlo Ersek wrote: Hi Laszlo, I'll respond first to this mail' I'll take my time with the rest :) > On 01/08/18 22:50, Marcel Apfelbaum wrote: >> When all the fw_cfg slots are used, a write is made outside the >> bounds of the fw_cfg files array as part of the sort algorithm. >> >> Fix it by avoiding an unnecessary array element move. >> Fix also an assert while at it. >> >> Signed-off-by: Marcel Apfelbaum >> --- >> hw/nvram/fw_cfg.c | 6 ++++-- >> 1 file changed, 4 insertions(+), 2 deletions(-) >> >> diff --git a/hw/nvram/fw_cfg.c b/hw/nvram/fw_cfg.c >> index 753ac0e4ea..4313484b21 100644 >> --- a/hw/nvram/fw_cfg.c >> +++ b/hw/nvram/fw_cfg.c >> @@ -784,7 +784,7 @@ void fw_cfg_add_file_callback(FWCfgState *s, cons= t char *filename, >> * index and "i - 1" is the one being copied from, thus the >> * unusual start and end in the for statement. >> */ >> - for (i =3D count + 1; i > index; i--) { >> + for (i =3D count; i > index; i--) { >> s->files->f[i] =3D s->files->f[i - 1]; >> s->files->f[i].select =3D cpu_to_be16(FW_CFG_FILE_FIRST + i)= ; >> s->entries[0][FW_CFG_FILE_FIRST + i] =3D >=20 > This hunk looks correct to me. After my change or before? I think I am right. At this point we have "count" elements in the array. That means the last element in the array is at arr[count - 1]. We want to make room for the new element at index, so we move all the elements from index to index + 1. The first element we should move is arr[count - 1] to arr[count]. But the code moved arr[count] to arr [count + 1]. This move is not needed. We currently have count elements in the > array, so we cannot normally access the element *at* count. However, we > are extending the array right now, therefore we can assign (store) the > element at count (and then we'll increment count later). But accessing > an element at (count+1) is wrong. >=20 >> @@ -833,7 +833,6 @@ void *fw_cfg_modify_file(FWCfgState *s, const char= *filename, >> assert(s->files); >> =20 >> index =3D be32_to_cpu(s->files->count); >> - assert(index < fw_cfg_file_slots(s)); >> =20 >> for (i =3D 0; i < index; i++) { >> if (strcmp(filename, s->files->f[i].name) =3D=3D 0) { >> @@ -843,6 +842,9 @@ void *fw_cfg_modify_file(FWCfgState *s, const char= *filename, >> return ptr; >> } >> } >> + >> + assert(index < fw_cfg_file_slots(s)); >> + >> /* add new one */ >> fw_cfg_add_file_callback(s, filename, NULL, NULL, NULL, data, le= n, true); >> return NULL; >> >=20 > I think I agree with Marc-Andr=C3=A9 here, when I say, replace the asse= rt > with a comment instead? (About the fact that fw_cfg_add_file_callback() > will assert(), *if* we reach that far.) Hmm, what should we add to the comment? "We lost, brace for impact :)" My point, if we are going to abort, let's abort as early as we can. But if is a consensus, I'll get rid of it. Thanks, Marcel >=20 > Thanks > Laszlo >=20