Hi Miles,
On 11/27/20 5:07 AM, Miles Chen wrote:
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().
Nit: s/usespace/userspace/ (please search and replace all occurrences :) )
I tested with 5.10-rc4 and the issue remains.
Explaination from Catalin in [1]:
Nit: s/Explaination/Explanation/ (please search and replace all occurrences :) )
" 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]:
Nit: s/baed/based/ (please search and replace all occurrences :) )
[...]
fs/proc/task_mmu.c | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-)
diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c index 217aa2705d5d..92b277388f05 100644 --- a/fs/proc/task_mmu.c +++ b/fs/proc/task_mmu.c @@ -1599,11 +1599,15 @@ static ssize_t pagemap_read(struct file *file, char __user *buf, 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))
It seems that 'svpfn' should be less-then-equal (<=) '(ULONG_MAX >> PAGE_SHIFT)'. Is there any specific reason why this is not the case?
start_vaddr = untagged_addr(svpfn << PAGE_SHIFT);- /* Ensure the address is inside the task */
- if (start_vaddr > mm->task_size) start_vaddr = end_vaddr;
/*
Otherwise:
Reviewed-by: Vincenzo Frascino vincenzo.frascino@arm.com