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

["Song Bao Hua (Barry Song)" <song.bao.hua@hisilicon.com>]

> -----Original Message-----
> From: Miles Chen [mailto:miles.chen@mediatek.com]
> Sent: Thursday, November 26, 2020 8:49 PM
> To: Song Bao Hua (Barry Song) <song.bao.hua@hisilicon.com>
> Cc: Alexey Dobriyan <adobriyan@gmail.com>; Andrew Morton
> <akpm@linux-foundation.org>; linux-kernel@vger.kernel.org;
> linux-fsdevel@vger.kernel.org; linux-mediatek@lists.infradead.org;
> wsd_upstream@mediatek.com
> Subject: RE: [RESEND PATCH v1] proc: use untagged_addr() for pagemap_read
> addresses
> 
> On Thu, 2020-11-26 at 07:16 +0000, Song Bao Hua (Barry Song) wrote:
> >
> > > -----Original Message-----
> > > From: Miles Chen [mailto:miles.chen@mediatek.com]
> > > Sent: Monday, November 23, 2020 7:39 PM
> > > To: Alexey Dobriyan <adobriyan@gmail.com>; Andrew Morton
> > > <akpm@linux-foundation.org>
> > > Cc: linux-kernel@vger.kernel.org; linux-fsdevel@vger.kernel.org;
> > > linux-mediatek@lists.infradead.org; wsd_upstream@mediatek.com; Miles
> > > Chen <miles.chen@mediatek.com>
> > > 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@mediatek.com>
> > > ---
> > >  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.
> >
> 
> 
> I run arm64 devices.
> If the tagged pointer is enabled, the ARM top-byte Ignore is also
> enabled. It means the top-byte is always be ignored.
> 
> I think the kernel should not break existing userspace program, so it's
> better to handle the tagged user pointer in kernel.
> 
> grep 'untag' mm -RnH to see existing examples.

I see your point. But the existing examples such as gup etc are receiving
an address from userspace.

pagemap isn't getting an address, it is getting a file offset which
should be figured out by userspace correctly. While we read a file,
we should make sure the offset is in the range of the file length.

> 
> 
> thanks,
> Miles
> 
> > >
> > >  	/*
> > > --
> > > 2.18.0
> >

Thanks
Barry