On Wed, Jan 06 2016 at 8:22pm -0500, Scotty Bauer <sba...@eng.utah.edu> wrote:
> > > On 01/05/2016 02:13 PM, Mike Snitzer wrote: > > On Tue, Jan 05 2016 at 3:16pm -0500, > > Mike Snitzer <snit...@redhat.com> wrote: > > > >> On Tue, Dec 08 2015 at 1:26pm -0500, > >> Scotty Bauer <sba...@eng.utah.edu> 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. You're fine, no worries. But I've just dropped the offending original commit from linux-next and it obviously won't be included in 4.5 I'll revisit whether we need to bother with the extra & change you're suggesting while coming to terms with why I was able to be lulled into thinking your original patch was correct ;) -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
