On Thu, Feb 28, 2019 at 1:57 PM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote: > On Thu, 28 Feb 2019, Jann Horn wrote: > > +Josh for unwinding, +x86 folks > > On Wed, Feb 27, 2019 at 11:43 PM Andrew Morton > > <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > > > On Thu, 21 Feb 2019 06:52:04 -0800 syzbot <syzbot+ca95b2b7aef9e7cbd6ab@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote: > > > > > > > Hello, > > > > > > > > syzbot found the following crash on: > > > > > > > > HEAD commit: 4aa9fc2a435a Revert "mm, memory_hotplug: initialize struct.. > > > > git tree: upstream > > > > console output: https://syzkaller.appspot.com/x/log.txt?x=1101382f400000 > > > > kernel config: https://syzkaller.appspot.com/x/.config?x=4fceea9e2d99ac20 > > > > dashboard link: https://syzkaller.appspot.com/bug?extid=ca95b2b7aef9e7cbd6ab > > > > compiler: gcc (GCC) 9.0.0 20181231 (experimental) > > > > > > > > Unfortunately, I don't have any reproducer for this crash yet. > > > > > > Not understanding. That seems to be saying that we got a NULL pointer > > > deref in __generic_file_write_iter() at > > > > > > written = generic_perform_write(file, from, iocb->ki_pos); > > > > > > which isn't possible. > > > > > > I'm not seeing recent changes in there which could have caused this. Help. > > > > + > > > > Maybe the problem is that the frame pointer unwinder isn't designed to > > cope with NULL function pointers - or more generally, with an > > unwinding operation that starts before the function's frame pointer > > has been set up? > > > > Unwinding starts at show_trace_log_lvl(). That begins with > > unwind_start(), which calls __unwind_start(), which uses > > get_frame_pointer(), which just returns regs->bp. But that frame > > pointer points to the part of the stack that's storing the address of > > the caller of the function that called NULL; the caller of NULL is > > skipped, as far as I can tell. > > > > What's kind of annoying here is that we don't have a proper frame set > > up yet, we only have half a stack frame (saved RIP but no saved RBP). > > That wreckage is related to the fact that the indirect calls are going > through __x86_indirect_thunk_$REG. I just verified on a VM with some other > callback NULL'ed that the resulting backtrace is not really helpful. > > So in that case generic_perform_write() has two indirect calls: > > mapping->a_ops->write_begin() and ->write_end() Does the indirect thunk thing really make any difference? When you arrive at RIP=NULL, RSP points to a saved instruction pointer, just like when indirect calls are compiled normally. I just compiled kernels with artificial calls to a NULL function pointer (in prctl_set_seccomp()), with retpoline disabled, with both unwinders. The ORC unwinder shows a call trace with "?" everywhere that doesn't show the caller: [ 228.219140] BUG: unable to handle kernel NULL pointer dereference at 0000000000000000 [ 228.223897] #PF error: [INSTR] [ 228.224562] PGD 0 P4D 0 [ 228.225119] Oops: 0010 [#1] PREEMPT SMP DEBUG_PAGEALLOC KASAN [ 228.226319] CPU: 1 PID: 1099 Comm: artificial_null Not tainted 5.0.0-rc8+ #299 [ 228.227818] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014 [ 228.229542] RIP: 0010: (null) [ 228.230331] Code: Bad RIP value. [ 228.231011] RSP: 0018:ffff8881d798fe88 EFLAGS: 00010246 [ 228.232104] RAX: ffffffffffffffda RBX: 0000000000000016 RCX: ffffffffa0368205 [ 228.233599] RDX: dffffc0000000000 RSI: 00007ffde0d71168 RDI: 0000000000000042 [ 228.235077] RBP: 1ffff1103af31fd4 R08: 0000561b50807740 R09: 0000000000000016 [ 228.236557] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000042 [ 228.238039] R13: 0000000000000000 R14: 00007ffde0d71168 R15: 0000561b50807740 [ 228.239517] FS: 00007fe31f1cf700(0000) GS:ffff8881eb040000(0000) knlGS:0000000000000000 [ 228.241213] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [ 228.242411] CR2: ffffffffffffffd6 CR3: 00000001df8b8004 CR4: 0000000000360ee0 [ 228.243886] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 [ 228.245364] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 [ 228.246841] Call Trace: [ 228.247366] ? __x64_sys_prctl+0x402/0x680 [ 228.248224] ? __ia32_sys_prctl+0x6e0/0x6e0 [ 228.249106] ? __do_page_fault+0x457/0x620 [ 228.249969] ? do_syscall_64+0x6d/0x160 [ 228.250778] ? entry_SYSCALL_64_after_hwframe+0x44/0xa9 [...] whereas the FP unwinder shows this, listing prctl_set_seccomp only with a question mark: [ 47.469957] BUG: unable to handle kernel NULL pointer dereference at 0000000000000000 [ 47.476973] #PF error: [INSTR] [ 47.477742] PGD 0 P4D 0 [ 47.478341] Oops: 0010 [#1] PREEMPT SMP DEBUG_PAGEALLOC KASAN [ 47.479703] CPU: 4 PID: 1079 Comm: artificial_null Not tainted 5.0.0-rc8+ #300 [ 47.481489] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014 [ 47.483442] RIP: 0010: (null) [ 47.484328] Code: Bad RIP value. [ 47.485085] RSP: 0018:ffff8881e01f7e70 EFLAGS: 00010246 [ 47.486358] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: ffffffffafbf007a [ 47.488090] RDX: dffffc0000000000 RSI: 00007ffe164b4f28 RDI: 0000000000000042 [ 47.489862] RBP: ffff8881e01f7e78 R08: 0000562942136740 R09: 0000000000000016 [ 47.491491] R10: 0000000000000000 R11: 0000000000000000 R12: 1ffff1103c03efd3 [ 47.493144] R13: 0000000000000042 R14: 00007ffe164b4f28 R15: 0000000000000016 [ 47.494795] FS: 00007fa38b1d6700(0000) GS:ffff8881eb300000(0000) knlGS:0000000000000000 [ 47.496638] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [ 47.497981] CR2: ffffffffffffffd6 CR3: 00000001e0e4c006 CR4: 0000000000360ee0 [ 47.499623] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 [ 47.501252] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 [ 47.502927] Call Trace: [ 47.503501] ? prctl_set_seccomp+0x3a/0x50 [ 47.504450] __x64_sys_prctl+0x457/0x6f0 [ 47.505349] ? __ia32_sys_prctl+0x750/0x750 [ 47.506352] do_syscall_64+0x72/0x160 [ 47.507214] entry_SYSCALL_64_after_hwframe+0x44/0xa9 [...] Looking back at the syzkaller report, the command line output (https://syzkaller.appspot.com/x/log.txt?x=1101382f400000) has this: [ 375.092788] Call Trace: [ 375.095378] ? generic_perform_write+0x2a0/0x6b0 [ 375.100150] ? add_page_wait_queue+0x480/0x480 [ 375.104744] ? current_time+0x1b0/0x1b0 [ 375.108727] ? generic_write_check_limits+0x380/0x380 [ 375.113942] ? ext4_file_write_iter+0x28b/0x1410 {some non-dmesg output here} [ 375.118711] __generic_file_write_iter+0x25e/0x630 [ 375.123714] ext4_file_write_iter+0x37a/0x1410 The first entry with a question mark is *RSP, the real caller; that's generic_perform_write(), as expected. The rest is probably just random garbage that happened to still be on the stack. It looks like syzkaller strips out trace elements with question marks in front. So I think this doesn't really have anything to do with __x86_indirect_thunk_$REG, and the best possible fix might be to teach the unwinders that RIP==NULL means "pretend that RIP is *real_RSP and that RSP is real_RSP+8, and report *real_RSP as the first element of the backtrace".
