Re: missing stack trace entry on NULL pointer call [was: Re: BUG: unable to handle kernel NULL pointer dereference in __generic_file_write_iter]
On Thu, Feb 28, 2019 at 5:34 PM Jann Horn wrote: > > On Thu, Feb 28, 2019 at 1:57 PM Thomas Gleixner 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 > > > wrote: > > > > On Thu, 21 Feb 2019 06:52:04 -0800 syzbot > > > > 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=1101382f40 > > > > > 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_$REG. I just verified on a VM with some other > > callback NULL'ed that the resulting backtrace is not really helpful. > > > > So in that case generic_perform_write() has two indirect calls: > > > > mapping->a_ops->write_begin() and ->write_end() > > Does the indirect thunk thing really make any difference? When you > arrive at RIP=NULL, RSP points to a saved instruction pointer, just > like when indirect calls are compiled normally. > > I just compiled kernels with artificial calls to a NULL function > pointer (in prctl_set_seccomp()), with retpoline disabled, with both > unwinders. The ORC unwinder shows a call trace with "?" everywhere > that doesn't show the caller: [...] > So I think this doesn't really have anything to do with > __x86_indirect_thunk_$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". Cooking up some patches now...
Re: missing stack trace entry on NULL pointer call [was: Re: BUG: unable to handle kernel NULL pointer dereference in __generic_file_write_iter]
On Thu, Feb 28, 2019 at 1:57 PM Thomas Gleixner 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 > > wrote: > > > On Thu, 21 Feb 2019 06:52:04 -0800 syzbot > > > 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=1101382f40 > > > > 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 [ 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: 0018:8881d798fe88 EFLAGS: 00010246 [ 228.232104] RAX: ffda RBX: 0016 RCX: a0368205 [ 228.233599] RDX: dc00 RSI: 7ffde0d71168 RDI: 0042 [ 228.235077] RBP: 11103af31fd4 R08: 561b50807740 R09: 0016 [ 228.236557] R10: R11: R12: 0042 [ 228.238039] R13: R14: 7ffde0d71168 R15: 561b50807740 [ 228.239517] FS: 7fe31f1cf700() GS:8881eb04() knlGS: [ 228.241213] CS: 0010 DS: ES: CR0: 80050033 [ 228.242411] CR2: ffd6 CR3: 0001df8b8004 CR4: 00360ee0 [ 228.243886] DR0: DR1: DR2: [ 228.245364] DR3: DR6: fffe0ff0 DR7: 0400 [ 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 [ 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: 0018:8881e01f7e70 EFLAGS: 00010246 [
Re: missing stack trace entry on NULL pointer call [was: Re: BUG: unable to handle kernel NULL pointer dereference in __generic_file_write_iter]
On Thu, 28 Feb 2019, Jann Horn wrote: > +Josh for unwinding, +x86 folks > On Wed, Feb 27, 2019 at 11:43 PM Andrew Morton > wrote: > > On Thu, 21 Feb 2019 06:52:04 -0800 syzbot > > 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=1101382f40 > > > 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() Thanks, tglx
missing stack trace entry on NULL pointer call [was: Re: BUG: unable to handle kernel NULL pointer dereference in __generic_file_write_iter]
+Josh for unwinding, +x86 folks On Wed, Feb 27, 2019 at 11:43 PM Andrew Morton wrote: > On Thu, 21 Feb 2019 06:52:04 -0800 syzbot > 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=1101382f40 > > 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). > > IMPORTANT: if you fix the bug, please add the following tag to the commit: > > Reported-by: syzbot+ca95b2b7aef9e7cbd...@syzkaller.appspotmail.com > > > > BUG: unable to handle kernel NULL pointer dereference at > > #PF error: [INSTR] > > PGD a7ea0067 P4D a7ea0067 PUD 81535067 PMD 0 > > Oops: 0010 [#1] PREEMPT SMP KASAN > > CPU: 0 PID: 15924 Comm: syz-executor0 Not tainted 5.0.0-rc4+ #50 > > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS > > Google 01/01/2011 > > RIP: 0010: (null) > > Code: Bad RIP value. > > RSP: 0018:88804c3d7858 EFLAGS: 00010246 > > RAX: RBX: 885aeb60 RCX: 005b > > RDX: RSI: 88807ec22930 RDI: 8880a59bdcc0 > > RBP: 88804c3d79b8 R08: R09: 88804c3d7910 > > R10: 8880835ca200 R11: R12: 005b > > R13: 88804c3d7c98 R14: dc00 R15: > > FS: 7f3456db4700() GS:8880ae60() knlGS: > > CS: 0010 DS: ES: CR0: 80050033 > > CR2: ffd6 CR3: 814ac000 CR4: 001406f0 > > DR0: DR1: DR2: > > DR3: DR6: fffe0ff0 DR7: 0400 > > Call Trace: > > __generic_file_write_iter+0x25e/0x630 mm/filemap.c: > > ext4_file_write_iter+0x37a/0x1410 fs/ext4/file.c:266 > > call_write_iter include/linux/fs.h:1862 [inline] > > new_sync_write fs/read_write.c:474 [inline] > > __vfs_write+0x764/0xb40 fs/read_write.c:487 > > vfs_write+0x20c/0x580 fs/read_write.c:549 > > ksys_write+0x105/0x260 fs/read_write.c:598 > > __do_sys_write fs/read_write.c:610 [inline] > > __se_sys_write fs/read_write.c:607 [inline] > > __x64_sys_write+0x73/0xb0 fs/read_write.c:607 > > do_syscall_64+0x1a3/0x800 arch/x86/entry/common.c:290 > > entry_SYSCALL_64_after_hwframe+0x49/0xbe > > RIP: 0033:0x458089 > > Code: 6d b7 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7 > > 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff > > ff 0f 83 3b b7 fb ff c3 66 2e 0f 1f 84 00 00 00 00 > > RSP: 002b:7f3456db3c78 EFLAGS: 0246 ORIG_RAX: 0001 > > RAX: ffda RBX: 0003 RCX: 00458089 > > RDX: 005b RSI: 2240 RDI: 0003 > > RBP: 0073bf00 R08: R09: > > R10: R11: 0246 R12: 7f3456db46d4 > > R13: 004c7450 R14: 004dce68 R15: > > Modules linked in: > > CR2: > > ---[ end trace 5cac9d2c75a59916 ]--- > > kobject: 'loop5' (4426a409): kobject_uevent_env > > RIP: 0010: (null) > > Code: Bad RIP value. > > RSP: 0018:88804c3d7858 EFLAGS: 00010246 > > RAX: RBX: 885aeb60 RCX: 005b > > kobject: 'loop5' (4426a409): fill_kobj_path: path > > = '/devices/virtual/block/loop5' > > RDX: RSI: 88807ec22930 RDI: 8880a59bdcc0 > > kobject: 'loop2' (b82e0c58): kobject_uevent_env > > kobject: 'loop2' (b82e0c58): fill_kobj_path: path > > = '/devices/virtual/block/loop2' > > RBP: 88804c3d79b8 R08:
