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

[David Hildenbrand <david@redhat.com>]

On 28.05.25 17:25, Peter Xu wrote:
> On Wed, May 28, 2025 at 05:02:15PM +0200, David Hildenbrand wrote:
>> On 28.05.25 16:54, Peter Xu wrote:
>>> [Add Jason]
>>>
>>> On Wed, May 28, 2025 at 11:59:56AM +0200, David Hildenbrand wrote:
>>>> 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.
> 
> [1]
> 
>>>
>>> We may want to be careful on this.
>>>
>>> I feel like we can still potentially break drivers that
>>> follow_pfnmap_start() used to work on debateable things like RAM page
>>> injections, unless breaking them is the intention.
>>
>> Yes, that all needs a cleanup likely; it's all very confusing and
>> inconsistent.
>>
>>>
>>> OTOH, I also see at least two in-tree drivers set VM_IO|VM_MIXEDMAP:
>>>
>>> *** drivers/gpu/drm/gma500/fbdev.c:
>>> psb_fbdev_fb_mmap[110]         vm_flags_set(vma, VM_IO | VM_MIXEDMAP | VM_DONTEXPAND | VM_DONTDUMP);
>>>
>>> *** drivers/gpu/drm/omapdrm/omap_gem.c:
>>> omap_gem_object_mmap[538]      vm_flags_set(vma, VM_DONTEXPAND | VM_DONTDUMP | VM_IO | VM_MIXEDMAP);
>>>
>>> AFAIU, these MIXEDMAP users will still rely on follow_pfnmap_start() to
>>> work on e.g. RAM pages, because GUP will simply fail them..
>>
>> Right.
>>
>> VM_IO essentially tells us "don't touch this memory, it might have side
>> effects", such as MMIO, that's why GUP outright refuses VM_IO VMAs.
>>
>> I am not sure why generic_access_phys() should be allowed to ... touch that
>> memory instead?
> 
> I'm looking at:
> 
> commit 28b2ee20c7cba812b6f2ccf6d722cf86d00a84dc
> Author: Rik van Riel <riel@redhat.com>
> Date:   Wed Jul 23 21:27:05 2008 -0700
> 
>      access_process_vm device memory infrastructure
> 
> VM_IO is also intentionally mentioned in the doc too:
> 
> Documentation/filesystems/locking.rst
> 
>          ->access() is called when get_user_pages() fails in
>          access_process_vm(), typically used to debug a process through
>          /proc/pid/mem or ptrace.  This function is needed only for
>          VM_IO | VM_PFNMAP VMAs.
> 
> So it definitely looks like intentional, though I know nothing about PPC
> Cell SPUs..


VM_IO | VM_PFNMAP, I can understand that. It's weird combined with weird.

But the use case for "VM_IO | VM_MIXEDMAP" ?

To be precise, I am questioning if follow_pfnmap_start() should only 
work on ...

PFNMAPs ?

:)

-- 
Cheers,

David / dhildenb