RE: [RESEND PATCH v1] proc: use untagged_addr() for pagemap_read addresses

[Song Bao Hua (Barry Song)]

> -----Original Message-----
> From: Miles Chen [mailto:miles.chen@xxxxxxxxxxxx]
> Sent: Monday, November 23, 2020 7:39 PM
> To: Alexey Dobriyan <adobriyan@xxxxxxxxx>; Andrew Morton
> <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx; linux-fsdevel@xxxxxxxxxxxxxxx;
> linux-mediatek@xxxxxxxxxxxxxxxxxxx; wsd_upstream@xxxxxxxxxxxx; Miles
> Chen <miles.chen@xxxxxxxxxxxx>
> Subject: [RESEND PATCH v1] 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.
> 
> My test code is baed on [1]:
> 
> 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://github.com/stressapptest/stressapptest/blob/master/src/os.cc#L158
> 
> Signed-off-by: Miles Chen <miles.chen@xxxxxxxxxxxx>
> ---
>  fs/proc/task_mmu.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 217aa2705d5d..e9a70f7ee515 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -1599,11 +1599,11 @@ static ssize_t pagemap_read(struct file *file, char
> __user *buf,
> 
>  	src = *ppos;
>  	svpfn = src / PM_ENTRY_BYTES;
> -	start_vaddr = svpfn << PAGE_SHIFT;
> +	start_vaddr = untagged_addr(svpfn << PAGE_SHIFT);
>  	end_vaddr = mm->task_size;
> 
>  	/* watch out for wraparound */
> -	if (svpfn > mm->task_size >> PAGE_SHIFT)
> +	if (start_vaddr > mm->task_size)
>  		start_vaddr = end_vaddr;

Wouldn't the untag be done by the user reading pagemap file?
With this patch, even users pass an illegal address, for example,
users put a tag on a virtual address which hasn't really a tag,
they will still get the right pagemap.

> 
>  	/*
> --
> 2.18.0

Thanks
Barry