From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:52432) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1eYu48-0001l9-MT for qemu-devel@nongnu.org; Tue, 09 Jan 2018 08:35:46 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1eYu43-000463-PO for qemu-devel@nongnu.org; Tue, 09 Jan 2018 08:35:44 -0500 From: Laszlo Ersek References: <20180108215007.46471-1-marcel@redhat.com> <3ea6176b-01a4-8f5a-81fc-3e9a8c846dc7@redhat.com> <2b536f93-4f84-c09d-2a65-a6c653999be3@redhat.com> Message-ID: <15eabb6d-0199-5bf3-678e-ffa97c352c72@redhat.com> Date: Tue, 9 Jan 2018 14:35:29 +0100 MIME-Version: 1.0 In-Reply-To: <2b536f93-4f84-c09d-2a65-a6c653999be3@redhat.com> Content-Type: text/plain; charset=utf-8 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: Marcel Apfelbaum , qemu-devel@nongnu.org Cc: ehabkost@redhat.com, mst@redhat.com, qemu-stable@nongnu.org, kraxel@redhat.com On 01/09/18 14:33, Laszlo Ersek wrote: > On 01/09/18 14:18, Marcel Apfelbaum wrote: >> 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 >>>> --- >>>> =C2=A0 hw/nvram/fw_cfg.c | 6 ++++-- >>>> =C2=A0 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,=C2=A0 >>>> const char *filename, >>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 * index and "i - 1" is the one = being copied from, thus the >>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 * unusual start and end in the = for statement. >>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 */ >>>> -=C2=A0=C2=A0=C2=A0 for (i =3D count + 1; i > index; i--) { >>>> +=C2=A0=C2=A0=C2=A0 for (i =3D count; i > index; i--) { >>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 s->files->f[i= ] =3D s->files->f[i - 1]; >>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 s->files->f[i= ].select =3D cpu_to_be16(FW_CFG_FILE_FIRST + i); >>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 s->entries[0]= [FW_CFG_FILE_FIRST + i] =3D >>> >>> This hunk looks correct to me. >> >> After my change or before? >=20 > Well, the source code doesn't have "hunks", patches have hunks. :) >=20 > So, I meant, this part of your patch was correct, IMO. >=20 >> >> 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. >> >> >> =C2=A0We 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) th= e >>> element at count (and then we'll increment count later). But accessin= g >>> an element at (count+1) is wrong. >>> >>>> @@ -833,7 +833,6 @@ void *fw_cfg_modify_file(FWCfgState *s, const >>>> char *filename, >>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 assert(s->files); >>>> =C2=A0 =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 index =3D be32_to_cpu(s->files= ->count); >>>> -=C2=A0=C2=A0=C2=A0 assert(index < fw_cfg_file_slots(s)); >>>> =C2=A0 =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 for (i =3D 0; i < index; i++) = { >>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (strcmp(fi= lename, s->files->f[i].name) =3D=3D 0) { >>>> @@ -843,6 +842,9 @@ void *fw_cfg_modify_file(FWCfgState *s, const >>>> char *filename, >>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0 return ptr; >>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 } >>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 } >>>> + >>>> +=C2=A0=C2=A0=C2=A0 assert(index < fw_cfg_file_slots(s)); >>>> + >>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 /* add new one */ >>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 fw_cfg_add_file_callback(s, filename,= NULL, NULL, NULL, data, >>>> len, true); >>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 return NULL; >>>> >>> >>> I think I agree with Marc-Andr=C3=A9 here, when I say, replace the as= sert >>> 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. >=20 > No, it's going to be another assert, just later. Assume that at this > point we have (index =3D=3D fw_cfg_file_slots(s)), because the function > didn't find the element to modify, so it decides to add a new one, but > also we do not have room for the new one. So, with the suggested remova= l > of the assert, we call fw_cfg_add_file_callback(). >=20 > Then, fw_cfg_add_file_callback() does: >=20 > if (!s->files) { > dsize =3D sizeof(uint32_t) + sizeof(FWCfgFile) * fw_cfg_file_sl= ots(s); > s->files =3D g_malloc0(dsize); > fw_cfg_add_bytes(s, FW_CFG_FILE_DIR, s->files, dsize); > } >=20 > count =3D be32_to_cpu(s->files->count); > assert(count < fw_cfg_file_slots(s)); >=20 > The (!s->files) condition is expected to eval to false (our table is > full, so we do have a table). >=20 > And then, the assert() below the "if" will fire. >=20 > Am I missing something? Hm, OK, your point was, abort as *early* as we can. I guess you are not wrong :) I'm fine either way, then. Thanks Laszlo