Re: + proc-use-untagged_addr-for-pagemap_read-addresses.patch added to -mm tree

[Miles Chen <miles.chen@mediatek.com>]

On Fri, 2020-11-27 at 19:52 -0800, akpm@linux-foundation.org wrote:
> The patch titled
>      Subject: proc: use untagged_addr() for pagemap_read addresses
> has been added to the -mm tree.  Its filename is
>      proc-use-untagged_addr-for-pagemap_read-addresses.patch
> 
> This patch should soon appear at
>     https://ozlabs.org/~akpm/mmots/broken-out/proc-use-untagged_addr-for-pagemap_read-addresses.patch
> and later at
>     https://ozlabs.org/~akpm/mmotm/broken-out/proc-use-untagged_addr-for-pagemap_read-addresses.patch
> 
> Before you just go and hit "reply", please:
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>    b) Prefer to cc a suitable mailing list as well
>    c) Ideally: find the original patch on the mailing list and do a
>       reply-to-all to that, adding suitable additional cc's
> 
> *** Remember to use Documentation/process/submit-checklist.rst when testing your code ***
> 
> The -mm tree is included into linux-next and is updated
> there every 3-4 working days

Sorry for bothering, I checked the next-20201201 tag and the patch in
next-20201201 is [1], but there is a overflow issue in [1] so I
submitted v2 [2]. We should take [2], right? 
(the patch in this email is [2] but the patch in next-20201201 is [1])

[1] https://lore.kernel.org/patchwork/patch/1343258/
[1] https://lore.kernel.org/patchwork/patch/1345874/



> 
> ------------------------------------------------------
> From: Miles Chen <miles.chen@mediatek.com>
> Subject: proc: use untagged_addr() for pagemap_read addresses
> 
> When we try to visit the pagemap of a tagged userspace pointer, we find
> that the start_vaddr is not correct because of the tag.  To fix it, we
> should untag the usespace pointers in pagemap_read().
> 
> I tested with 5.10-rc4 and the issue remains.
> 
> Explanation from Catalin in [1]:
> 
> : Arguably, that's a user-space bug since tagged file offsets were never
> : supported.  In this case it's not even a tag at bit 56 as per the arm64
> : tagged address ABI but rather down to bit 47.  You could say that the
> : problem is caused by the C library (malloc()) or whoever created the
> : tagged vaddr and passed it to this function.  It's not a kernel regression
> : as we've never supported it.
> : 
> : Now, pagemap is a special case where the offset is usually not generated
> : as a classic file offset but rather derived by shifting a user virtual
> : address.  I guess we can make a concession for pagemap (only) and allow
> : such offset with the tag at bit (56 - PAGE_SHIFT + 3).
> 
> My test code is baed on [2]:
> 
> A userspace pointer which has been tagged by 0xb4: 0xb400007662f541c8
> 
> === userspace program ===
> 
> uint64 OsLayer::VirtualToPhysical(void *vaddr) {
> 	uint64 frame, paddr, pfnmask, pagemask;
> 	int pagesize = sysconf(_SC_PAGESIZE);
> 	off64_t off = ((uintptr_t)vaddr) / pagesize * 8; // off = 0xb400007662f541c8 / pagesize * 8 = 0x5a00003b317aa0
> 	int fd = open(kPagemapPath, O_RDONLY);
> 	...
> 
> 	if (lseek64(fd, off, SEEK_SET) != off || read(fd, &frame, 8) != 8) {
> 		int err = errno;
> 		string errtxt = ErrorString(err);
> 		if (fd >= 0)
> 			close(fd);
> 		return 0;
> 	}
> ...
> }
> 
> === kernel fs/proc/task_mmu.c ===
> 
> static ssize_t pagemap_read(struct file *file, char __user *buf,
> 		size_t count, loff_t *ppos)
> {
> 	...
> 	src = *ppos;
> 	svpfn = src / PM_ENTRY_BYTES; // svpfn == 0xb400007662f54
> 	start_vaddr = svpfn << PAGE_SHIFT; // start_vaddr == 0xb400007662f54000
> 	end_vaddr = mm->task_size;
> 
> 	/* watch out for wraparound */
> 	// svpfn == 0xb400007662f54
> 	// (mm->task_size >> PAGE) == 0x8000000
> 	if (svpfn > mm->task_size >> PAGE_SHIFT) // the condition is true because of the tag 0xb4
> 		start_vaddr = end_vaddr;
> 
> 	ret = 0;
> 	while (count && (start_vaddr < end_vaddr)) { // we cannot visit correct entry because start_vaddr is set to end_vaddr
> 		int len;
> 		unsigned long end;
> 		...
> 	}
> 	...
> }
> 
> [1] https://lore.kernel.org/patchwork/patch/1343258/
> [2] https://github.com/stressapptest/stressapptest/blob/master/src/os.cc#L158
> 
> Link: https://lkml.kernel.org/r/20201127050738.14440-1-miles.chen@mediatek.com
> Signed-off-by: Miles Chen <miles.chen@mediatek.com>
> Cc: Alexey Dobriyan <adobriyan@gmail.com>
> Cc: Andrey Konovalov <andreyknvl@google.com>
> Cc: Alexander Potapenko <glider@google.com>
> Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
> Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Dmitry Vyukov <dvyukov@google.com>
> Cc: Marco Elver <elver@google.com>
> Cc: Will Deacon <will.deacon@arm.com>
> Cc: Eric W. Biederman <ebiederm@xmission.com>
> Cc: Song Bao Hua (Barry Song) <song.bao.hua@hisilicon.com>
> Cc: <stable@vger.kernel.org>	[5.4+]
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> ---
> 
>  fs/proc/task_mmu.c |    8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> --- a/fs/proc/task_mmu.c~proc-use-untagged_addr-for-pagemap_read-addresses
> +++ a/fs/proc/task_mmu.c
> @@ -1599,11 +1599,15 @@ static ssize_t pagemap_read(struct file
>  
>  	src = *ppos;
>  	svpfn = src / PM_ENTRY_BYTES;
> -	start_vaddr = svpfn << PAGE_SHIFT;
>  	end_vaddr = mm->task_size;
>  
>  	/* watch out for wraparound */
> -	if (svpfn > mm->task_size >> PAGE_SHIFT)
> +	start_vaddr = end_vaddr;
> +	if (svpfn < (ULONG_MAX >> PAGE_SHIFT))
> +		start_vaddr = untagged_addr(svpfn << PAGE_SHIFT);
> +
> +	/* Ensure the address is inside the task */
> +	if (start_vaddr > mm->task_size)
>  		start_vaddr = end_vaddr;
>  
>  	/*
> _
> 
> Patches currently in -mm which might be from miles.chen@mediatek.com are
> 
> proc-use-untagged_addr-for-pagemap_read-addresses.patch
>