Re: [PATCH] mm: fix COW mapping handing in generic_access_phys

[David Hildenbrand <david@redhat.com>]

On 28.05.25 10:59, David Hildenbrand wrote:
> On 28.05.25 03:56, Jinjiang Tu wrote:
>> Syzkaller reports a below BUG:
>>    ioremap on RAM at 0x0000000022727000 - 0x0000000022727fff
>>    WARNING: CPU: 3 PID: 3609 at arch/x86/mm/ioremap.c:216 __ioremap_caller+0x644/0x7f0 arch/x86/mm/ioremap.c:216
>>    Modules linked in:
>>    CPU: 3 PID: 3609 Comm: syz.2.577 Not tainted 6.6.0+ #63
>>    Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014
>>    RIP: 0010:__ioremap_caller+0x644/0x7f0 arch/x86/mm/ioremap.c:216
>>    Call Trace:
>>     <TASK>
>>     generic_access_phys+0x241/0x480 mm/memory.c:6458
>>     __access_remote_vm+0x6af/0x970 mm/memory.c:6535
>>     access_process_vm+0x53/0x80 mm/memory.c:6600
>>     get_cmdline+0x192/0x380 mm/util.c:1041
>>     audit_log_proctitle kernel/auditsc.c:1620 [inline]
>>     audit_log_exit+0x1424/0x18c0 kernel/auditsc.c:1811
>>     __audit_syscall_exit+0x252/0x2f0 kernel/auditsc.c:2079
>>     audit_syscall_exit include/linux/audit.h:356 [inline]
>>     syscall_exit_work+0x10f/0x130 kernel/entry/common.c:166
>>     __syscall_exit_to_user_mode_work kernel/entry/common.c:205 [inline]
>>     syscall_exit_to_user_mode+0x10/0x1e0 kernel/entry/common.c:218
>>     do_syscall_64+0x66/0x110 arch/x86/entry/common.c:87
>>     entry_SYSCALL_64_after_hwframe+0x78/0xe2
>>
>> The /dev/mem is mapped with COW mapping, and mremap at the mm->args_start.
>> The special pfn mapping is replaced by anon folios due to COW.
>> generic_access_phys() is supposed to handle iomem, instead of RAM pfn,
>> thus trigger a WARN_ON.
>>
>> Similar to commit 04c35ab3bdae ("x86/mm/pat: fix VM_PAT handling in
>> COW mappings"). check if the pte is special to reject Cowed anon folios.
>>
>> Signed-off-by: Jinjiang Tu <tujinjiang@huawei.com>
>> ---
>>    mm/memory.c | 7 +++++++
>>    1 file changed, 7 insertions(+)
>>
>> diff --git a/mm/memory.c b/mm/memory.c
>> index 49199410805c..e1dac84536ee 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -6840,6 +6840,13 @@ int generic_access_phys(struct vm_area_struct *vma, unsigned long addr,
>>    retry:
>>    	if (follow_pfnmap_start(&args))
>>    		return -EINVAL;
>> +
>> +	/* Never return PFNs of anon folios in COW mappings. */
>> +	if (!args.special) {
>> +		follow_pfnmap_end(&args);
>> +		return -EINVAL;
>> +	}
>> +
>>    	prot = args.pgprot;
>>    	phys_addr = (resource_size_t)args.pfn << PAGE_SHIFT;
>>    	writable = args.writable;
> 
> I assume we trigger this through vma->vm_ops->access, when the vm_ops have generic_access_phys set.
> 
> I still dislike exposing the "special" bit here, as it is absolutely not what we should care about in the caller.
> 
> In case our arch does not support pte_special, you fix will not catch that case ...
> 
> The following might be better:
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 37d8738f5e12e..810adb8d1a53b 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -6681,6 +6681,14 @@ int generic_access_phys(struct vm_area_struct *vma, unsigned long addr,
>           prot = args.pgprot;
>           phys_addr = (resource_size_t)args.pfn << PAGE_SHIFT;
>           writable = args.writable;
> +
> +       /* Refuse (refcounted) anonymous pages in CoW mappings. */
> +       if (is_cow_mapping(vma->vm_flags) &&
> +           vm_normal_page(vma, addr, ptep_get(args.ptep))) {
> +               follow_pfnmap_end(&args);
> +               return -EINVAL;
> +       }
> +

Thinking again, we might have a PMD/PUD mapping, so maybe 
follow_pfnmap_start() should really just refuse any refcounted pages.

-- 
Cheers,

David / dhildenb