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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id s22sm103042884pfh.107.2019.08.08.13.42.00 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Thu, 08 Aug 2019 13:42:00 -0700 (PDT) Date: Thu, 8 Aug 2019 13:41:59 -0700 From: Kees Cook To: Al Viro Cc: Matthew Wilcox , syzbot , allison@lohutok.net, andreyknvl@google.com, cai@lca.pw, gregkh@linuxfoundation.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-usb@vger.kernel.org, syzkaller-bugs@googlegroups.com, tglx@linutronix.de, Jiri Kosina Subject: Re: BUG: bad usercopy in hidraw_ioctl Message-ID: <201908081330.98485D9@keescook> References: <000000000000ce6527058f8bf0d0@google.com> <20190807195821.GD5482@bombadil.infradead.org> <20190808014925.GL1131@ZenIV.linux.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190808014925.GL1131@ZenIV.linux.org.uk> X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Thu, Aug 08, 2019 at 02:49:25AM +0100, Al Viro wrote: > On Wed, Aug 07, 2019 at 12:58:21PM -0700, Matthew Wilcox wrote: > > On Wed, Aug 07, 2019 at 12:28:06PM -0700, syzbot wrote: > > > usercopy: Kernel memory exposure attempt detected from wrapped address > > > (offset 0, size 0)! > > > ------------[ cut here ]------------ > > > kernel BUG at mm/usercopy.c:98! > > > > This report is confusing because the arguments to usercopy_abort() are wrong. > > > > /* Reject if object wraps past end of memory. */ > > if (ptr + n < ptr) > > usercopy_abort("wrapped address", NULL, to_user, 0, ptr + n); (Just to reiterate for this branch of the thread: this is an off-by-one false positive already fixed in -mm for -next. However, see below...) > > > > ptr + n is not 'size', it's what wrapped. I don't know what 'offset' > > should be set to, but 'size' should be 'n'. Presumably we don't want to > > report 'ptr' because it'll leak a kernel address ... reporting 'n' will > > leak a range for a kernel address, but I think that's OK? Admittedly an > > attacker can pass in various values for 'n', but it'll be quite noisy > > and leave a trace in the kernel logs for forensics to find afterwards. > > > > > Call Trace: > > > check_bogus_address mm/usercopy.c:151 [inline] > > > __check_object_size mm/usercopy.c:260 [inline] > > > __check_object_size.cold+0xb2/0xba mm/usercopy.c:250 > > > check_object_size include/linux/thread_info.h:119 [inline] > > > check_copy_size include/linux/thread_info.h:150 [inline] > > > copy_to_user include/linux/uaccess.h:151 [inline] > > > hidraw_ioctl+0x38c/0xae0 drivers/hid/hidraw.c:392 > > > > The root problem would appear to be: > > > > else if (copy_to_user(user_arg + offsetof( > > struct hidraw_report_descriptor, > > value[0]), > > dev->hid->rdesc, > > min(dev->hid->rsize, len))) > > > > That 'min' should surely be a 'max'? > > Surely not. ->rsize is the amount of data available to copy out; len > is the size of buffer supplied by userland to copy into. include/uapi/linux/hid.h:#define HID_MAX_DESCRIPTOR_SIZE 4096 drivers/hid/hidraw.c: if (get_user(len, (int __user *)arg)) ret = -EFAULT; else if (len > HID_MAX_DESCRIPTOR_SIZE - 1) ret = -EINVAL; else if (copy_to_user(user_arg + offsetof( struct hidraw_report_descriptor, value[0]), dev->hid->rdesc, min(dev->hid->rsize, len))) ret = -EFAULT; The copy size must be less than 4096, which means dev->hid->rdesc is allocated at the highest page of memory. That whole space collides with the ERR_PTR region which has two bad potential side-effects: 1) something that checks for ERR_PTRs combined with a high allocation will think it failed and leak the allocation. 2) something that doesn't check ERR_PTRs might try to stomp on an actual allocation in that area. How/why is there memory allocated there, I thought it was intentionally left unused specifically for ERR_PTR: Documentation/x86/x86_64/mm.rst: Start addr | Offset | End addr | Size | VM area description ========================================================================== ... ffffffffffe00000 | -2 MB | ffffffffffffffff | 2 MB | ...unused hole or is this still a real bug with an invalid dev->hid->rdesc which was about to fault but usercopy got in the way first? -- Kees Cook