Re: [PATCH v2 2/5] x86/mm: Populate KASAN shadow for entire per-CPU range of CPU entry area

[Andrey Ryabinin <ryabinin.a.a@gmail.com>]

On 11/10/22 23:35, Sean Christopherson wrote:
> Populate a KASAN shadow for the entire possible per-CPU range of the CPU
> entry area instead of requiring that each individual chunk map a shadow.
> Mapping shadows individually is error prone, e.g. the per-CPU GDT mapping
> was left behind, which can lead to not-present page faults during KASAN
> validation if the kernel performs a software lookup into the GDT.  The DS
> buffer is also likely affected.
> 
> The motivation for mapping the per-CPU areas on-demand was to avoid
> mapping the entire 512GiB range that's reserved for the CPU entry area,
> shaving a few bytes by not creating shadows for potentially unused memory
> was not a goal.
> 
> The bug is most easily reproduced by doing a sigreturn with a garbage
> CS in the sigcontext, e.g.
> 
>   int main(void)
>   {
>     struct sigcontext regs;
> 
>     syscall(__NR_mmap, 0x1ffff000ul, 0x1000ul, 0ul, 0x32ul, -1, 0ul);
>     syscall(__NR_mmap, 0x20000000ul, 0x1000000ul, 7ul, 0x32ul, -1, 0ul);
>     syscall(__NR_mmap, 0x21000000ul, 0x1000ul, 0ul, 0x32ul, -1, 0ul);
> 
>     memset(&regs, 0, sizeof(regs));
>     regs.cs = 0x1d0;
>     syscall(__NR_rt_sigreturn);
>     return 0;
>   }
> 
> to coerce the kernel into doing a GDT lookup to compute CS.base when
> reading the instruction bytes on the subsequent #GP to determine whether
> or not the #GP is something the kernel should handle, e.g. to fixup UMIP
> violations or to emulate CLI/STI for IOPL=3 applications.
> 
>   BUG: unable to handle page fault for address: fffffbc8379ace00
>   #PF: supervisor read access in kernel mode
>   #PF: error_code(0x0000) - not-present page
>   PGD 16c03a067 P4D 16c03a067 PUD 15b990067 PMD 15b98f067 PTE 0
>   Oops: 0000 [#1] PREEMPT SMP KASAN
>   CPU: 3 PID: 851 Comm: r2 Not tainted 6.1.0-rc3-next-20221103+ #432
>   Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
>   RIP: 0010:kasan_check_range+0xdf/0x190
>   Call Trace:
>    <TASK>
>    get_desc+0xb0/0x1d0
>    insn_get_seg_base+0x104/0x270
>    insn_fetch_from_user+0x66/0x80
>    fixup_umip_exception+0xb1/0x530
>    exc_general_protection+0x181/0x210
>    asm_exc_general_protection+0x22/0x30
>   RIP: 0003:0x0
>   Code: Unable to access opcode bytes at 0xffffffffffffffd6.
>   RSP: 0003:0000000000000000 EFLAGS: 00000202
>   RAX: 0000000000000000 RBX: 0000000000000000 RCX: 00000000000001d0
>   RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
>   RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
>   R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
>   R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
>    </TASK>
> 
> Fixes: 9fd429c28073 ("x86/kasan: Map shadow for percpu pages on demand")
> Reported-by: syzbot+ffb4f000dc2872c93f62@syzkaller.appspotmail.com
> Suggested-by: Andrey Ryabinin <ryabinin.a.a@gmail.com>
> Cc: Alexander Potapenko <glider@google.com>
> Cc: Andrey Konovalov <andreyknvl@gmail.com>
> Cc: Dmitry Vyukov <dvyukov@google.com>
> Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
> Cc: kasan-dev@googlegroups.com
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>  arch/x86/mm/cpu_entry_area.c | 8 +++-----
>  1 file changed, 3 insertions(+), 5 deletions(-)
> 

Reviewed-by: Andrey Ryabinin <ryabinin.a.a@gmail.com>