Re: [PATCH v3] arm64: mm: fix pass user prot to ioremap_prot in generic_access_phys

On Fri, Jan 30, 2026 at 03:38:07PM +0800, Jinjiang Tu wrote:

Here is a syzkaller error log:
  [0000000020ffc000] pgd=080000010598d403, p4d=080000010598d403, pud=0800000125ddb403,
                     pmd=080000007833c403, pte=01608000007fcfcf
  Unable to handle kernel read from unreadable memory at virtual address ffff80008ea89000
  KASAN: probably user-memory-access in range [0x0000000475448000-0x0000000475448007]
  Mem abort info:
    ESR = 0x000000009600000f
    EC = 0x25: DABT (current EL), IL = 32 bits
    SET = 0, FnV = 0
    EA = 0, S1PTW = 0
    FSC = 0x0f: level 3 permission fault
  Data abort info:
    ISV = 0, ISS = 0x0000000f, ISS2 = 0x00000000
    CM = 0, WnR = 0, TnD = 0, TagAccess = 0
    GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0
  swapper pgtable: 4k pages, 48-bit VAs, pgdp=00000001244aa000
  [ffff80008ea89000] pgd=100000013ffff403, p4d=100000013ffff403, pud=100000013fffe403,
		     pmd=100000010a453403, pte=01608000007fcfcf
  Internal error: Oops: 000000009600000f [#1] SMP
  Modules linked in: team
  CPU: 1 PID: 10840 Comm: syz.9.83 Kdump: loaded Tainted: G
  Hardware name: QEMU KVM Virtual Machine, BIOS 0.0.0 02/06/2015
  pstate: 20400005 (nzCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
  pc : __memcpy_fromio+0x80/0xf8
  lr : generic_access_phys+0x20c/0x2b8
  sp : ffff8000a0507960
  x29: ffff8000a0507960 x28: 1ffff000140a0f44 x27: ffff00003833cfe0
  x26: 0000000000000000 x25: 0000000000001000 x24: 0010000000000001
  x23: ffff80008ea89000 x22: ffff00004ea63000 x21: 0000000000001000
  x20: ffff80008ea89000 x19: ffff00004ea62000 x18: 0000000000000000
  x17: 0000000000000000 x16: 0000000000000000 x15: ffff8000806f1e3c
  x14: ffff8000806f1d44 x13: 0000000041b58ab3 x12: ffff7000140a0f23
  x11: 1ffff000140a0f22 x10: ffff7000140a0f22 x9 : ffff800080579d24
  x8 : 0000000000000004 x7 : 0000000000000003 x6 : 0000000000000001
  x5 : ffff8000a0507910 x4 : ffff7000140a0f22 x3 : dfff800000000000
  x2 : 0000000000001000 x1 : ffff80008ea89000 x0 : ffff00004ea62000
  Call trace:
    __memcpy_fromio+0x80/0xf8
    generic_access_phys+0x20c/0x2b8
    __access_remote_vm+0x46c/0x5b8
    access_remote_vm+0x18/0x30
    environ_read+0x238/0x3e8
    vfs_read+0xe4/0x2b0
    ksys_read+0xcc/0x178
    __arm64_sys_read+0x4c/0x68
    invoke_syscall+0x68/0x1a0
    el0_svc_common.constprop.0+0x11c/0x150
    do_el0_svc+0x38/0x50
    el0_svc+0x50/0x258
    el0t_64_sync_handler+0xc0/0xc8
    el0t_64_sync+0x1a4/0x1a8
  Code: 91002339 aa1403f7 8b190276 d503201f (f94002f8)

The local syzkaller first maps I/O address from /dev/mem to userspace,
overiding the stack vma with MAP_FIXED flag. As a result, when reading
/proc/$pid/environ, generic_access_phys() is called to access the region,
which triggers a PAN permission-check fault and causes a kernel access
fault.

The root cause is that generic_access_phys() passes a user pte to
ioremap_prot(), the user pte sets PTE_USER and PTE_NG bits.  Consequently,
any subsequent kernel-mode access to the remapped address raises a fault.

To fix it, define arch_mk_kernel_prot() to convert user prot to kernel
prot for arm64, and call arch_mk_kernel_prot() in generic_access_phys(),
so that a user prot is passed to ioremap_prot().

Fixes: 893dea9ccd08 ("arm64: Add HAVE_IOREMAP_PROT support")
Signed-off-by: Zeng Heng <redacted>
Signed-off-by: Jinjiang Tu <redacted>

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>

That's not urgent for 6.19, it's just misuse of /dev/mem (aren't they
all) but it's worth fixing.

Thanks.

-- 
Catalin