Hi Mr. Verkuil, On Wed, Aug 19, 2020 at 04:26:28PM +0200, Hans Verkuil wrote: > Hi Peilin, > > On 10/08/2020 07:05, Peilin Ye wrote: > > precalculate_color() is reading out of `sin` since `tpg->hue` is not being > > properly checked. Fix it. `cos` is safe, as long as `tpg->hue` is higher > > than or equal to -192. > > Thank you for this patch, but there is something I don't understand, namely > just *how* tpg->hue can be out-of-range. > > From what I can see vivid sets hue via tpg_s_hue() when the V4L2_CID_HUE control > is set. But that control has a range of -128...128, so ctrl->val should always be in > that range. > > I would really like to know 1) what the value of tpg->hue actually is when it goes > out of range, and 2) who is changing it to that value. Can you do a bit more digging? The value of `tpg->hue` was -20551. It came from the userspace, see the "\xb9\xaf" on line 500 of the reproducer: https://syzkaller.appspot.com/text?tag=ReproC&x=14b49e71e00000 NONFAILING(memcpy((void*)0x20000200, "/dev/video6\000\000", 13)); res = syscall(__NR_openat, 0xffffffffffffff9cul, 0x20000200ul, 2ul, 0ul); if (res != -1) r[0] = res; NONFAILING(memcpy((void*)0x20000140, "\x4d\x43\x66\x34\xfd\x89\xb9\xaf\x0d\x59\xa2\x83\x4c\xfd" ^^^^ ^^^^ ^^^^ ^^^^^^^^ "\x3e\x64\x7c\x96\xcd\x59\xf2\x3a\x18\xa3\x81\x49\x22\xc0" "\xc1\xbf\x02\xa5\x50\x5f\xcb\x48\x92\x0e\xf3\xdc\xff\x85" "\xb7\x84\x21\xab\xef\x31\x3d\xb1\xb6\x5d\xbf\x07\x8e\xee" "\x5e\x7c\x73\x32\xf4\x9d\x1e\x62\x6b\x6a\xa0\x74\x73\xe6" "\xca\x1b\xdb\x7a\xca\x76\xd8\x37\xb8\xd9", 80)); syscall(__NR_write, r[0], 0x20000140ul, 8ul); I guess the root cause is a race condition in the vivid test driver, which completely corrupted `tpg`. I see bytes like "\x4d", "\x66" and "\xfd" around `tpg->hue`, too. The reproducer does two things: the above write() on /dev/video6, and a preadv() on /dev/video3: NONFAILING(*(uint64_t*)0x20000800 = 0x20000000); NONFAILING(*(uint64_t*)0x20000808 = 0x1f); NONFAILING(*(uint64_t*)0x20000810 = 0); NONFAILING(*(uint64_t*)0x20000818 = 0); NONFAILING(*(uint64_t*)0x20000820 = 0); NONFAILING(*(uint64_t*)0x20000828 = 0); NONFAILING(*(uint64_t*)0x20000830 = 0); NONFAILING(*(uint64_t*)0x20000838 = 0); NONFAILING(*(uint64_t*)0x20000840 = 0); NONFAILING(*(uint64_t*)0x20000848 = 0); syscall(__NR_preadv, r[1], 0x20000800ul, 5ul, 0ul); I commented out this preadv(), then the reproducer didn't cause any crash. Unfortunately I don't know the code well enough in order to figure out exactly why...At this point of time I'd like to send you an v2 as you suggested, it should work as a mitigation. Thank you for the suggestion! Peilin Ye
