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* [PATCH] x86/mm: Populate KASAN shadow for per-CPU GDT mapping in CPU entry area
@ 2022-11-04 21:24 Sean Christopherson
  2022-11-08 20:14 ` Andrey Ryabinin
  0 siblings, 1 reply; 2+ messages in thread
From: Sean Christopherson @ 2022-11-04 21:24 UTC (permalink / raw)
  To: Dave Hansen, Andy Lutomirski, Peter Zijlstra, Thomas Gleixner,
	Ingo Molnar, Borislav Petkov, x86
  Cc: H. Peter Anvin, linux-kernel, syzbot+ffb4f000dc2872c93f62,
	Andrey Ryabinin, Alexander Potapenko, Andrey Konovalov,
	Dmitry Vyukov, Vincenzo Frascino, kasan-dev, Sean Christopherson

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;
    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:
  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

Fixes: 9fd429c28073 ("x86/kasan: Map shadow for percpu pages on demand")
Cc: Andrey Ryabinin <>
Cc: Alexander Potapenko <>
Cc: Andrey Konovalov <>
Cc: Dmitry Vyukov <>
Cc: Vincenzo Frascino <>
Signed-off-by: Sean Christopherson <>
 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;
-	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);
 			     per_cpu_ptr(&entry_stack_storage, cpu), 1,

base-commit: 81214a573d19ae2fa5b528286ba23cd1cb17feec

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2022-11-04 21:24 [PATCH] x86/mm: Populate KASAN shadow for per-CPU GDT mapping in CPU entry area Sean Christopherson
2022-11-08 20:14 ` Andrey Ryabinin

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