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

[Will Deacon]

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 <zengheng4 at huawei.com>
> Signed-off-by: Jinjiang Tu <tujinjiang at huawei.com>
> ---
> Changes in v3:
>  * arch_mk_kernel_prot() always grant read/write permissions.
> 
>  arch/arm64/include/asm/io.h |  8 ++++++++
>  mm/memory.c                 | 12 +++++++++++-
>  2 files changed, 19 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/include/asm/io.h b/arch/arm64/include/asm/io.h
> index 83e03abbb2ca..fe3040d59119 100644
> --- a/arch/arm64/include/asm/io.h
> +++ b/arch/arm64/include/asm/io.h
> @@ -267,6 +267,14 @@ int arm64_ioremap_prot_hook_register(const ioremap_prot_hook_t hook);
>  
>  #define ioremap_prot ioremap_prot
>  
> +#define arch_mk_kernel_prot arch_mk_kernel_prot
> +static inline pgprot_t arch_mk_kernel_prot(pgprot_t user_prot)
> +{
> +	ptdesc_t mem_type = pgprot_val(user_prot) & PTE_ATTRINDX_MASK;
> +
> +	return __pgprot_modify(PAGE_KERNEL, PTE_ATTRINDX_MASK, mem_type);
> +}

Do we really need another arch helper here?

It looks to me like generic_access_phys() is the only caller of
ioremap_prot() outside of arch/, so why not just handle the user_prot in
there on arm64? You could rename the existing ioremap_prot() function in
arch/arm64/mm/ioremap.c to e.g. __ioremap_prot(), switch over the
callers in arch/arm64 and then add ioremap_prot() to wrap that with your
pgprot modification above.

Will