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;
Eric
