[PATCH] mm: mempolicy: fix potential pte_unmap_unlock pte error

[PATCH] mm: mempolicy: fix potential pte_unmap_unlock pte error

[Shijie Luo]

When flags in queue_pages_pte_range don't have MPOL_MF_MOVE or MPOL_MF_MOVE_ALL
 bits, code breaks and passing origin pte - 1 to pte_unmap_unlock seems like
not a good idea.

queue_pages_pte_range can run in MPOL_MF_MOVE_ALL mode which doesn't migrate
misplaced pages but returns with EIO when encountering such a page. Since
commit a7f40cfe3b7a ("mm: mempolicy: make mbind() return -EIO when MPOL_MF_STRICT
 is specified") and early break on the first pte in the range results in
pte_unmap_unlock on an underflow pte. This can lead to lockups later on when
 somebody  tries to lock the pte resp. page_table_lock again..

Fixes: a7f40cfe3b7a ("mm: mempolicy: make mbind() return -EIO when
MPOL_MF_STRICT is specified")

Signed-off-by: Shijie Luo <luoshijie1@huawei.com>
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
---
 mm/mempolicy.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 3fde772ef5ef..3ca4898f3f24 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -525,7 +525,7 @@ static int queue_pages_pte_range(pmd_t *pmd, unsigned long addr,
 	unsigned long flags = qp->flags;
 	int ret;
 	bool has_unmovable = false;
-	pte_t *pte;
+	pte_t *pte, *mapped_pte;
 	spinlock_t *ptl;
 
 	ptl = pmd_trans_huge_lock(pmd, vma);
@@ -539,7 +539,7 @@ static int queue_pages_pte_range(pmd_t *pmd, unsigned long addr,
 	if (pmd_trans_unstable(pmd))
 		return 0;
 
-	pte = pte_offset_map_lock(walk->mm, pmd, addr, &ptl);
+	mapped_pte = pte = pte_offset_map_lock(walk->mm, pmd, addr, &ptl);
 	for (; addr != end; pte++, addr += PAGE_SIZE) {
 		if (!pte_present(*pte))
 			continue;
@@ -571,7 +571,7 @@ static int queue_pages_pte_range(pmd_t *pmd, unsigned long addr,
 		} else
 			break;
 	}
-	pte_unmap_unlock(pte - 1, ptl);
+	pte_unmap_unlock(mapped_pte, ptl);
 	cond_resched();
 
 	if (has_unmovable)
-- 
2.19.1

Re: [PATCH] mm: mempolicy: fix potential pte_unmap_unlock pte error

[Oscar Salvador]

On Mon, Oct 19, 2020 at 03:48:53AM -0400, Shijie Luo wrote:
> When flags in queue_pages_pte_range don't have MPOL_MF_MOVE or MPOL_MF_MOVE_ALL
>  bits, code breaks and passing origin pte - 1 to pte_unmap_unlock seems like
> not a good idea.

I think the above is already explained below?

> queue_pages_pte_range can run in MPOL_MF_MOVE_ALL mode which doesn't migrate
> misplaced pages but returns with EIO when encountering such a page. Since
> commit a7f40cfe3b7a ("mm: mempolicy: make mbind() return -EIO when MPOL_MF_STRICT
>  is specified") and early break on the first pte in the range results in
> pte_unmap_unlock on an underflow pte. This can lead to lockups later on when
>  somebody  tries to lock the pte resp. page_table_lock again..
> 
> Fixes: a7f40cfe3b7a ("mm: mempolicy: make mbind() return -EIO when
> MPOL_MF_STRICT is specified")
> 
> Signed-off-by: Shijie Luo <luoshijie1@huawei.com>
> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>

Anyway, LGTM:

Reviewed-by: Oscar Salvador <osalvador@suse.de>

-- 
Oscar Salvador
SUSE L3

Re: [PATCH] mm: mempolicy: fix potential pte_unmap_unlock pte error

[Michal Hocko]

On Mon 19-10-20 12:50:34, Oscar Salvador wrote:
> On Mon, Oct 19, 2020 at 03:48:53AM -0400, Shijie Luo wrote:
> > When flags in queue_pages_pte_range don't have MPOL_MF_MOVE or MPOL_MF_MOVE_ALL
> >  bits, code breaks and passing origin pte - 1 to pte_unmap_unlock seems like
> > not a good idea.
> 
> I think the above is already explained below?

Yes

> > queue_pages_pte_range can run in MPOL_MF_MOVE_ALL mode which doesn't migrate
> > misplaced pages but returns with EIO when encountering such a page. Since
> > commit a7f40cfe3b7a ("mm: mempolicy: make mbind() return -EIO when MPOL_MF_STRICT
> >  is specified") and early break on the first pte in the range results in
> > pte_unmap_unlock on an underflow pte. This can lead to lockups later on when
> >  somebody  tries to lock the pte resp. page_table_lock again..
> > 
> > Fixes: a7f40cfe3b7a ("mm: mempolicy: make mbind() return -EIO when
> > MPOL_MF_STRICT is specified")

Cc: stable

is due as well. There are even security concerns and I wouldn't be
surprised if this gained a CVE.

> > Signed-off-by: Shijie Luo <luoshijie1@huawei.com>
> > Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
> 
> Anyway, LGTM:
> 
> Reviewed-by: Oscar Salvador <osalvador@suse.de>

Acked-by: Michal Hocko <mhocko@suse.com>
-- 
Michal Hocko
SUSE Labs