Re: [PATCH] x86/mm: Populate KASAN shadow for per-CPU GDT mapping in CPU entry area

[Andrey Ryabinin]

On 11/5/22 00:24, Sean Christopherson wrote:
> Bounce through cea_map_percpu_pages() when setting protections for the
> per-CPU GDT mapping so that KASAN populates a shadow for said mapping.
> Failure to populate the shadow will result in a not-present #PF during
> KASAN validation if the kernel performs a software lookup into the GDT.
> 
> 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@xxxxxxxxxxxxxxxxxxxxxxxxx
> Cc: Andrey Ryabinin <ryabinin.a.a@xxxxxxxxx>
> Cc: Alexander Potapenko <glider@xxxxxxxxxx>
> Cc: Andrey Konovalov <andreyknvl@xxxxxxxxx>
> Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Cc: Vincenzo Frascino <vincenzo.frascino@xxxxxxx>
> Cc: kasan-dev@xxxxxxxxxxxxxxxx
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  arch/x86/mm/cpu_entry_area.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/mm/cpu_entry_area.c b/arch/x86/mm/cpu_entry_area.c
> index dff9001e5e12..4a6440461c10 100644
> --- a/arch/x86/mm/cpu_entry_area.c
> +++ b/arch/x86/mm/cpu_entry_area.c
> @@ -195,7 +195,7 @@ static void __init setup_cpu_entry_area(unsigned int cpu)
>  	pgprot_t tss_prot = PAGE_KERNEL;
>  #endif
>  
> -	cea_set_pte(&cea->gdt, get_cpu_gdt_paddr(cpu), gdt_prot);
> +	cea_map_percpu_pages(&cea->gdt, get_cpu_gdt_rw(cpu), 1, gdt_prot);


I'm thinking using kasan_populate_shadow_for_vaddr() in cea_map_percpu_page() wasn't the right idea.
We should just map shadow for entire 'cea' from setup_cpu_entry_area() instead of fixing it up in random places.
I mean like this:

---
 arch/x86/mm/cpu_entry_area.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/x86/mm/cpu_entry_area.c b/arch/x86/mm/cpu_entry_area.c
index dff9001e5e12..b122fa5e805b 100644
--- a/arch/x86/mm/cpu_entry_area.c
+++ b/arch/x86/mm/cpu_entry_area.c
@@ -195,6 +195,9 @@ static void __init setup_cpu_entry_area(unsigned int cpu)
 	pgprot_t tss_prot = PAGE_KERNEL;
 #endif
 
+	kasan_populate_shadow_for_vaddr(cea, CPU_ENTRY_AREA_SIZE,
+					early_cpu_to_node(cpu));
+
 	cea_set_pte(&cea->gdt, get_cpu_gdt_paddr(cpu), gdt_prot);
 
 	cea_map_percpu_pages(&cea->entry_stack_page,
-- 
2.37.4