On Tue, 2020-11-24 at 12:32 -0600, Eric W. Biederman wrote:
> Miles Chen <miles.chen@xxxxxxxxxxxx> writes:
>
> > 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
>
>
> Sigh this patch is buggy.
>
> > === 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);
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>
> Arguably the line above is safe, but unfortunately it has the
> possibility of suffering from overflow.
>
> > 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;
>
> Overflow handling you are removing here.
> >
> > /*
>
>
> I suspect the proper way to handle this is to move the test for
> overflow earlier so the code looks something like:
>
> end_vaddr = mm->task_size;
>
> src = *ppos;
> svpfn = src / PM_ENTRY_BYTES;
>
> /* watch out for wraparound */
> 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;
Thanks for the comment, I will fix that in patch v2.
Miles
>
> Eric
>
