On 2025/2/11 20:09, David Hildenbrand wrote:
On 11.02.25 10:43, Qi Zheng wrote:
On 2025/2/11 17:37, David Hildenbrand wrote:
On 11.02.25 10:29, Qi Zheng wrote:
On 2025/2/11 17:14, David Hildenbrand wrote:
On 11.02.25 04:45, Qi Zheng wrote:
Hi Russell,
On 2025/2/11 01:03, Russell King (Oracle) wrote:
On Mon, Feb 10, 2025 at 05:49:38PM +0100, Ezra Buehler wrote:
When running vanilla Linux 6.13 or newer (6.14-rc2) on the
AT91SAM9G25-based GARDENA smart Gateway, we are seeing a NULL
pointer
dereference resulting in a kernel panic. The culprit seems to be
commit
fc9c45b71f43 ("arm: adjust_pte() usepte_offset_map_rw_nolock()").
Reverting the commit apparently fixes the issue.
The blamed commit is buggy:
arch/arm/include/asm/tlbflush.h:
#define update_mmu_cache(vma, addr, ptep) \
update_mmu_cache_range(NULL, vma, addr, ptep, 1)
So vmf can be NULL. This didn't used to matter before this commit,
because vmf was not used by ARM's update_mmu_cache_range(). However,
the commit introduced a dereference of it, which now causes a NULL
point dereference.
Not sure what the correct solution is, but at a guess, both:
if (ptl != vmf->ptl)
need to become:
if (!vmf || ptl != vmf->ptl)
No, we can't do that, because without using split PTE locks, we would
use shared mm->page_table_lock, which would create a deadlock.
Maybe we can simply special-case on CONFIG_SPLIT_PTE_PTLOCKS ?
if (IS_ENABLED(CONFIG_SPLIT_PTE_PTLOCKS)) {
In this case, if two vmas map the same PTE page, then the same PTE lock
will be held repeatedly. Right?
Hmm, the comment says:
/*
* This is called while another page table is mapped, so we
* must use the nested version. This also means we need to
* open-code the spin-locking.
*/
"another page table" implies that it cannot be the same. But maybe that
comment was also wrong?
I don't see make_coherent() ensuring this when traversing vma.
Right, we could just have the same file range mapped MAP_SHARED into the
same page table using two VMAs ... I suspect writing a reproducer for
the deadlock should be easy.
I guess so, but I don't have an arm32 test environment yet. :(
[...]
vma_interval_tree_foreach(mpnt, &mapping->i_mmap, pgoff,
pgoff) {
+ unsigned long mpnt_addr;
+
/*
* If this VMA is not in our MM, we can ignore it.
* Note that we intentionally mask out the VMA
@@ -151,7 +178,14 @@ make_coherent(struct address_space *mapping, struct
vm_area_struct *vma,
if (!(mpnt->vm_flags & VM_MAYSHARE))
continue;
offset = (pgoff - mpnt->vm_pgoff) << PAGE_SHIFT;
- aliases += adjust_pte(mpnt, mpnt->vm_start + offset,
pfn, vmf);
+ mpnt_addr = mpnt->vm_start + offset;
+ /*
+ * If mpnt_addr and addr are mapped to the same PTE page,
+ * also skip this vma.
+ */
+ if (mpnt_addr >= start && mpnt_addr - start < PMD_SIZE)
+ continue;
Hmm, but is skipping the right thing to do? Maybe you would want to
Ah, your concern is that in this case we may also need to maintain cache
coherency. I'm not sure about this.
indicate to adjust_pte() whether it must take the PTL or not.
Indeed, this is a better approach. Will do it, and hope arm people
can help test it.
Thanks!
I did not study that code in detail, though ...
