From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752694AbcAGBW6 (ORCPT ); Wed, 6 Jan 2016 20:22:58 -0500 Received: from mailhub.eng.utah.edu ([155.98.110.27]:54538 "EHLO mailhub.eng.utah.edu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752253AbcAGBWx (ORCPT ); Wed, 6 Jan 2016 20:22:53 -0500 Subject: Re: dm ioctl: Access user-land memory through safe functions. To: Mike Snitzer References: <565DE2B8.7040709@eng.utah.edu> <566720D0.8080509@eng.utah.edu> <20160105201649.GA30512@redhat.com> <20160105211310.GC30512@redhat.com> Cc: dm-devel@redhat.com, linux-kernel@vger.kernel.org, agk@redhat.com, jmoyer@redhat.com From: Scotty Bauer X-Enigmail-Draft-Status: N1110 Message-ID: <568DBDDF.50708@eng.utah.edu> Date: Wed, 6 Jan 2016 18:22:39 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.4.0 MIME-Version: 1.0 In-Reply-To: <20160105211310.GC30512@redhat.com> Content-Type: multipart/mixed; boundary="------------020004000808080902030600" X-UCE-Score: -1.9 (-) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org This is a multi-part message in MIME format. --------------020004000808080902030600 Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit On 01/05/2016 02:13 PM, Mike Snitzer wrote: > On Tue, Jan 05 2016 at 3:16pm -0500, > Mike Snitzer wrote: > >> On Tue, Dec 08 2015 at 1:26pm -0500, >> Scotty Bauer wrote: >> >>> Friendly ping, is anyone interested in this? >> >> The passed @user argument is flagged via __user so it can be >> deferenced directly. It does look like directly deferencing >> user->version is wrong. >> >> But even if such indirect access is needed (because __user flag is only >> applicable to @user arg, not the contained version member) we could more >> easily just do something like this no?: >> >> uint32_t __user *versionp = (uint32_t __user *)user->version; >> ... >> if (copy_from_user(version, versionp, sizeof(version))) >> return -EFAULT; >> >> I've staged the following, thanks: >> https://git.kernel.org/cgit/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-4.5&id=bffc9e237a0c3176712bcd93fc6a184a61e0df26 > > Alasdair helped me understand that we do need your original fix. > I've staged it for 4.5 (and stable@) here: > https://git.kernel.org/cgit/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-4.5&id=ead3db62bf10fe143bec99e7b7ff370d7a6d23ef > > Thanks again, > Mike > -- This broke linux-next because I'm dumb and didn't test it. I thought it was a trivial enough of a patch that I wouldn't screw it up, but I did. I incorrectly assumed that user->version was essentially a pointer in userland, not a flat chunk of memory. Ie it was a pointer to some malloc'd region, not an inlined version[3]. I thought it was this: struct dm_ioctl { uint32_t *version; ... } It is really this: struct dm_ioctl { uint32_t version[3]; } I was trying to get the values out of *version, which would have been a pointer, but instead what the code ended up doing was actually getting 8 bytes of the version (think 4,3,1) out and trying to access that version as a memory address, oops. It turns out that the original code is correct and doesn't actually touch user memory without a copy_from_user(). Gcc is smart enough to see that version[3] is inlined, and it can emit code which simply takes the userland pointer (struct dm_ioctl __user user), and calculates on offset based on the pointer, thus no actual user dereference occurs. Had the struct looked like the first example I believe the patch would work. I'm wondering now if we should switch the code a bit to make it less ambiguous, so someone like me doesn't come along again thinking the code dereferences userland memory and waste everyones time. I've attached a patch based off linux-next-20150616 which reverts my broken code but adds an & to the front of user->version so it looks like the code is doing the right thing. If I should be basing my patch off something other than linux-next let me know and I'll rewrite it, or we can just revert the old patch and ignore this one. Thanks and very sorry for the confusion and breakage. --------------020004000808080902030600 Content-Type: text/x-patch; name="0001-dm-ioctl-disambiguate-the-user-pointer-calculation.patch" Content-Transfer-Encoding: 7bit Content-Disposition: attachment; filename*0="0001-dm-ioctl-disambiguate-the-user-pointer-calculation.patc"; filename*1="h" >>From 7dde54b74e4543b6f03ceb57f9479a1d402a3fd1 Mon Sep 17 00:00:00 2001 From: Scotty Bauer Date: Wed, 6 Jan 2016 18:17:35 -0700 Subject: [PATCH] dm ioctl: disambiguate the user pointer calculation This patch adds an & in front of user->version, in hopes of making it clear that user-memory is not being touched. Signed-off-by: Scotty Bauer --- drivers/md/dm-ioctl.c | 8 ++------ 1 file changed, 2 insertions(+), 6 deletions(-) diff --git a/drivers/md/dm-ioctl.c b/drivers/md/dm-ioctl.c index fa5bf54..81190df 100644 --- a/drivers/md/dm-ioctl.c +++ b/drivers/md/dm-ioctl.c @@ -1642,13 +1642,9 @@ static ioctl_fn lookup_ioctl(unsigned int cmd, int *ioctl_flags) static int check_version(unsigned int cmd, struct dm_ioctl __user *user) { uint32_t version[3]; - uint32_t __user *versionp; int r = 0; - if (copy_from_user(&versionp, &user->version, sizeof(versionp))) - return -EFAULT; - - if (copy_from_user(version, versionp, sizeof(version))) + if (copy_from_user(version, &user->version, sizeof(version))) return -EFAULT; if ((DM_VERSION_MAJOR != version[0]) || @@ -1667,7 +1663,7 @@ static int check_version(unsigned int cmd, struct dm_ioctl __user *user) version[0] = DM_VERSION_MAJOR; version[1] = DM_VERSION_MINOR; version[2] = DM_VERSION_PATCHLEVEL; - if (copy_to_user(versionp, version, sizeof(version))) + if (copy_to_user(&user->version, version, sizeof(version))) return -EFAULT; return r; -- 1.9.1 --------------020004000808080902030600-- From mboxrd@z Thu Jan 1 00:00:00 1970 From: Scotty Bauer Subject: Re: dm ioctl: Access user-land memory through safe functions. Date: Wed, 6 Jan 2016 18:22:39 -0700 Message-ID: <568DBDDF.50708@eng.utah.edu> References: <565DE2B8.7040709@eng.utah.edu> <566720D0.8080509@eng.utah.edu> <20160105201649.GA30512@redhat.com> <20160105211310.GC30512@redhat.com> Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="------------020004000808080902030600" Return-path: In-Reply-To: <20160105211310.GC30512@redhat.com> Sender: linux-kernel-owner@vger.kernel.org To: Mike Snitzer Cc: dm-devel@redhat.com, linux-kernel@vger.kernel.org, agk@redhat.com, jmoyer@redhat.com List-Id: dm-devel.ids This is a multi-part message in MIME format. --------------020004000808080902030600 Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit On 01/05/2016 02:13 PM, Mike Snitzer wrote: > On Tue, Jan 05 2016 at 3:16pm -0500, > Mike Snitzer wrote: > >> On Tue, Dec 08 2015 at 1:26pm -0500, >> Scotty Bauer wrote: >> >>> Friendly ping, is anyone interested in this? >> >> The passed @user argument is flagged via __user so it can be >> deferenced directly. It does look like directly deferencing >> user->version is wrong. >> >> But even if such indirect access is needed (because __user flag is only >> applicable to @user arg, not the contained version member) we could more >> easily just do something like this no?: >> >> uint32_t __user *versionp = (uint32_t __user *)user->version; >> ... >> if (copy_from_user(version, versionp, sizeof(version))) >> return -EFAULT; >> >> I've staged the following, thanks: >> https://git.kernel.org/cgit/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-4.5&id=bffc9e237a0c3176712bcd93fc6a184a61e0df26 > > Alasdair helped me understand that we do need your original fix. > I've staged it for 4.5 (and stable@) here: > https://git.kernel.org/cgit/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-4.5&id=ead3db62bf10fe143bec99e7b7ff370d7a6d23ef > > Thanks again, > Mike > -- This broke linux-next because I'm dumb and didn't test it. I thought it was a trivial enough of a patch that I wouldn't screw it up, but I did. I incorrectly assumed that user->version was essentially a pointer in userland, not a flat chunk of memory. Ie it was a pointer to some malloc'd region, not an inlined version[3]. I thought it was this: struct dm_ioctl { uint32_t *version; ... } It is really this: struct dm_ioctl { uint32_t version[3]; } I was trying to get the values out of *version, which would have been a pointer, but instead what the code ended up doing was actually getting 8 bytes of the version (think 4,3,1) out and trying to access that version as a memory address, oops. It turns out that the original code is correct and doesn't actually touch user memory without a copy_from_user(). Gcc is smart enough to see that version[3] is inlined, and it can emit code which simply takes the userland pointer (struct dm_ioctl __user user), and calculates on offset based on the pointer, thus no actual user dereference occurs. Had the struct looked like the first example I believe the patch would work. I'm wondering now if we should switch the code a bit to make it less ambiguous, so someone like me doesn't come along again thinking the code dereferences userland memory and waste everyones time. I've attached a patch based off linux-next-20150616 which reverts my broken code but adds an & to the front of user->version so it looks like the code is doing the right thing. If I should be basing my patch off something other than linux-next let me know and I'll rewrite it, or we can just revert the old patch and ignore this one. Thanks and very sorry for the confusion and breakage. --------------020004000808080902030600 Content-Type: text/x-patch; name="0001-dm-ioctl-disambiguate-the-user-pointer-calculation.patch" Content-Transfer-Encoding: 7bit Content-Disposition: attachment; filename*0="0001-dm-ioctl-disambiguate-the-user-pointer-calculation.patc"; filename*1="h" >From 7dde54b74e4543b6f03ceb57f9479a1d402a3fd1 Mon Sep 17 00:00:00 2001 From: Scotty Bauer Date: Wed, 6 Jan 2016 18:17:35 -0700 Subject: [PATCH] dm ioctl: disambiguate the user pointer calculation This patch adds an & in front of user->version, in hopes of making it clear that user-memory is not being touched. Signed-off-by: Scotty Bauer --- drivers/md/dm-ioctl.c | 8 ++------ 1 file changed, 2 insertions(+), 6 deletions(-) diff --git a/drivers/md/dm-ioctl.c b/drivers/md/dm-ioctl.c index fa5bf54..81190df 100644 --- a/drivers/md/dm-ioctl.c +++ b/drivers/md/dm-ioctl.c @@ -1642,13 +1642,9 @@ static ioctl_fn lookup_ioctl(unsigned int cmd, int *ioctl_flags) static int check_version(unsigned int cmd, struct dm_ioctl __user *user) { uint32_t version[3]; - uint32_t __user *versionp; int r = 0; - if (copy_from_user(&versionp, &user->version, sizeof(versionp))) - return -EFAULT; - - if (copy_from_user(version, versionp, sizeof(version))) + if (copy_from_user(version, &user->version, sizeof(version))) return -EFAULT; if ((DM_VERSION_MAJOR != version[0]) || @@ -1667,7 +1663,7 @@ static int check_version(unsigned int cmd, struct dm_ioctl __user *user) version[0] = DM_VERSION_MAJOR; version[1] = DM_VERSION_MINOR; version[2] = DM_VERSION_PATCHLEVEL; - if (copy_to_user(versionp, version, sizeof(version))) + if (copy_to_user(&user->version, version, sizeof(version))) return -EFAULT; return r; -- 1.9.1 --------------020004000808080902030600--