On Mon, Dec 09, 2024 at 02:35:27PM +0100, Jann Horn wrote: > On Mon, Dec 9, 2024 at 1:53 PM Lorenzo Stoakes > <lorenzo.stoakes@xxxxxxxxxx> wrote: > > On Mon, Dec 09, 2024 at 03:20:19AM -0800, syzbot wrote: > > > Hello, > > > > > > syzbot found the following issue on: > > > > > > HEAD commit: feffde684ac2 Merge tag 'for-6.13-rc1-tag' of git://git.ker.. > > > git tree: upstream > > > console output: https://syzkaller.appspot.com/x/log.txt?x=17f85fc0580000 > > > kernel config: https://syzkaller.appspot.com/x/.config?x=50c7a61469ce77e7 > > > dashboard link: https://syzkaller.appspot.com/bug?extid=2d788f4f7cb660dac4b7 > > > compiler: Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40 > > > > > > Unfortunately, I don't have any reproducer for this issue yet. > > > > Points to this being racey. > > > > > > > > Downloadable assets: > > > disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/7feb34a89c2a/non_bootable_disk-feffde68.raw.xz > > > vmlinux: https://storage.googleapis.com/syzbot-assets/6135c7297e8e/vmlinux-feffde68.xz > > > kernel image: https://storage.googleapis.com/syzbot-assets/6c154fdcc9cb/bzImage-feffde68.xz > > > > > > IMPORTANT: if you fix the issue, please add the following tag to the commit: > > > Reported-by: syzbot+2d788f4f7cb660dac4b7@xxxxxxxxxxxxxxxxxxxxxxxxx > > > > > > Oops: general protection fault, probably for non-canonical address 0xdffffc0000000080: 0000 [#1] PREEMPT SMP KASAN NOPTI > > > KASAN: null-ptr-deref in range [0x0000000000000400-0x0000000000000407] > > > > This doesn't make a huge amount of sense to me, the VMA is not 0x400 (1,024) > > bytes in size... and the actual faulting offset seems to be 0xdffffc0000000080 > > which is 0x80 off from some KASAN-specified value? > > If you look at the disassembly, you can see this: > > 13: 4d 89 ec mov %r13,%r12 > 16: 49 c1 ec 03 shr $0x3,%r12 > 1a: 48 b8 00 00 00 00 00 movabs $0xdffffc0000000000,%rax > 21: fc ff df > * 24: 41 80 3c 04 00 cmpb $0x0,(%r12,%rax,1) <-- trapping instruction > > R13 is 0000000000000406, that's the address we're about to access. > This code is trying to read KASAN shadow memory for that address by > reading from 0xdffffc0000000000+address>>3, which for real kernel > addresses gives you an address in the "KASAN shadow memory" range (see > https://kernel.org/doc/html/latest/arch/x86/x86_64/mm.html), but for > addresses in the low half of the address space gives you non-canonical > addresses starting with 0xdfff that cause #GP on access. > The second line "KASAN: null-ptr-deref in range > [0x0000000000000400-0x0000000000000407]" is basically computed by > doing that calculation in reverse. Ah thanks. > > > This would be vma->vm_file. But that also doesn't really make any sense. > > > > But I wonder... > > > > I see in the report at [0] that there's a failure injection in vm_area_dup() on > > fork: > > > > [ 73.842623][ T5318] ? kmem_cache_alloc_noprof+0x48/0x380 > > [ 73.844725][ T5318] ? __pfx___might_resched+0x10/0x10 > > [ 73.846687][ T5318] should_fail_ex+0x3b0/0x4e0 > > [ 73.848496][ T5318] should_failslab+0xac/0x100 > > [ 73.850232][ T5318] ? vm_area_dup+0x27/0x290 > > [ 73.852017][ T5318] kmem_cache_alloc_noprof+0x70/0x380 > > [ 73.854011][ T5318] vm_area_dup+0x27/0x290 > > [ 73.855771][ T5318] copy_mm+0xc1d/0x1f90 > > > > I also see in the fork logic we have the following code on error path: > > > > mas_set_range(&vmi.mas, mpnt->vm_start, mpnt->vm_end - 1); > > mas_store(&vmi.mas, XA_ZERO_ENTRY); > > > > And XA_ZERO_ENTRY is 0x406. > > That matches... And I wasn't aware that R13 was equal to the _actual_ address derefenced, really useful to know, I mentioned it in my mega reply where I figured out how we end up trying to deref this... :) yes I think this confirms the theory.
