| Message ID | 20240429114017.182570-1-ryan.roberts@arm.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | [v1] fs/proc/task_mmu: Fix loss of young/dirty bits during pagemap scan | expand |
On 29.04.24 13:40, Ryan Roberts wrote: > make_uffd_wp_pte() was previously doing: > > pte = ptep_get(ptep); > ptep_modify_prot_start(ptep); > pte = pte_mkuffd_wp(pte); > ptep_modify_prot_commit(ptep, pte); > > But if another thread accessed or dirtied the pte between the first 2 > calls, this could lead to loss of that information. Since > ptep_modify_prot_start() gets and clears atomically, the following is > the correct pattern and prevents any possible race. Any access after the > first call would see an invalid pte and cause a fault: > > pte = ptep_modify_prot_start(ptep); > pte = pte_mkuffd_wp(pte); > ptep_modify_prot_commit(ptep, pte); > > Fixes: 52526ca7fdb9 ("fs/proc/task_mmu: implement IOCTL to get and optionally clear info about PTEs") > Signed-off-by: Ryan Roberts <ryan.roberts@arm.com> > --- > fs/proc/task_mmu.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c > index 23fbab954c20..af4bc1da0c01 100644 > --- a/fs/proc/task_mmu.c > +++ b/fs/proc/task_mmu.c > @@ -1825,7 +1825,7 @@ static void make_uffd_wp_pte(struct vm_area_struct *vma, > pte_t old_pte; > > old_pte = ptep_modify_prot_start(vma, addr, pte); > - ptent = pte_mkuffd_wp(ptent); > + ptent = pte_mkuffd_wp(old_pte); > ptep_modify_prot_commit(vma, addr, pte, old_pte, ptent); Acked-by: David Hildenbrand <david@redhat.com>
Thanks! On 4/29/24 4:40 PM, Ryan Roberts wrote: > make_uffd_wp_pte() was previously doing: > > pte = ptep_get(ptep); > ptep_modify_prot_start(ptep); > pte = pte_mkuffd_wp(pte); > ptep_modify_prot_commit(ptep, pte); > > But if another thread accessed or dirtied the pte between the first 2 > calls, this could lead to loss of that information. Since > ptep_modify_prot_start() gets and clears atomically, the following is > the correct pattern and prevents any possible race. Any access after the > first call would see an invalid pte and cause a fault: > > pte = ptep_modify_prot_start(ptep); > pte = pte_mkuffd_wp(pte); > ptep_modify_prot_commit(ptep, pte); > > Fixes: 52526ca7fdb9 ("fs/proc/task_mmu: implement IOCTL to get and optionally clear info about PTEs") > Signed-off-by: Ryan Roberts <ryan.roberts@arm.com> Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com> Tested-by: Muhammad Usama Anjum <usama.anjum@collabora.com> > --- > fs/proc/task_mmu.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c > index 23fbab954c20..af4bc1da0c01 100644 > --- a/fs/proc/task_mmu.c > +++ b/fs/proc/task_mmu.c > @@ -1825,7 +1825,7 @@ static void make_uffd_wp_pte(struct vm_area_struct *vma, > pte_t old_pte; > > old_pte = ptep_modify_prot_start(vma, addr, pte); > - ptent = pte_mkuffd_wp(ptent); > + ptent = pte_mkuffd_wp(old_pte); > ptep_modify_prot_commit(vma, addr, pte, old_pte, ptent); > } else if (is_swap_pte(ptent)) { > ptent = pte_swp_mkuffd_wp(ptent); > -- > 2.25.1 >
On 29/04/2024 12:40, Ryan Roberts wrote: > make_uffd_wp_pte() was previously doing: > > pte = ptep_get(ptep); > ptep_modify_prot_start(ptep); > pte = pte_mkuffd_wp(pte); > ptep_modify_prot_commit(ptep, pte); > > But if another thread accessed or dirtied the pte between the first 2 > calls, this could lead to loss of that information. Since > ptep_modify_prot_start() gets and clears atomically, the following is > the correct pattern and prevents any possible race. Any access after the > first call would see an invalid pte and cause a fault: > > pte = ptep_modify_prot_start(ptep); > pte = pte_mkuffd_wp(pte); > ptep_modify_prot_commit(ptep, pte); > > Fixes: 52526ca7fdb9 ("fs/proc/task_mmu: implement IOCTL to get and optionally clear info about PTEs") > Signed-off-by: Ryan Roberts <ryan.roberts@arm.com> I guess this should have cc'ed stable but I forgot to add it. Are you able to fix this up when you take it, Andrew, or do I need to repost? > --- > fs/proc/task_mmu.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c > index 23fbab954c20..af4bc1da0c01 100644 > --- a/fs/proc/task_mmu.c > +++ b/fs/proc/task_mmu.c > @@ -1825,7 +1825,7 @@ static void make_uffd_wp_pte(struct vm_area_struct *vma, > pte_t old_pte; > > old_pte = ptep_modify_prot_start(vma, addr, pte); > - ptent = pte_mkuffd_wp(ptent); > + ptent = pte_mkuffd_wp(old_pte); > ptep_modify_prot_commit(vma, addr, pte, old_pte, ptent); > } else if (is_swap_pte(ptent)) { > ptent = pte_swp_mkuffd_wp(ptent); > -- > 2.25.1 >
On Mon, 29 Apr 2024 16:55:07 +0100 Ryan Roberts <ryan.roberts@arm.com> wrote: > > Fixes: 52526ca7fdb9 ("fs/proc/task_mmu: implement IOCTL to get and optionally clear info about PTEs") > > Signed-off-by: Ryan Roberts <ryan.roberts@arm.com> > > I guess this should have cc'ed stable but I forgot to add it. Are you able to > fix this up when you take it, Andrew, or do I need to repost? I have made that change.
diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c index 23fbab954c20..af4bc1da0c01 100644 --- a/fs/proc/task_mmu.c +++ b/fs/proc/task_mmu.c @@ -1825,7 +1825,7 @@ static void make_uffd_wp_pte(struct vm_area_struct *vma, pte_t old_pte; old_pte = ptep_modify_prot_start(vma, addr, pte); - ptent = pte_mkuffd_wp(ptent); + ptent = pte_mkuffd_wp(old_pte); ptep_modify_prot_commit(vma, addr, pte, old_pte, ptent); } else if (is_swap_pte(ptent)) { ptent = pte_swp_mkuffd_wp(ptent);
make_uffd_wp_pte() was previously doing: pte = ptep_get(ptep); ptep_modify_prot_start(ptep); pte = pte_mkuffd_wp(pte); ptep_modify_prot_commit(ptep, pte); But if another thread accessed or dirtied the pte between the first 2 calls, this could lead to loss of that information. Since ptep_modify_prot_start() gets and clears atomically, the following is the correct pattern and prevents any possible race. Any access after the first call would see an invalid pte and cause a fault: pte = ptep_modify_prot_start(ptep); pte = pte_mkuffd_wp(pte); ptep_modify_prot_commit(ptep, pte); Fixes: 52526ca7fdb9 ("fs/proc/task_mmu: implement IOCTL to get and optionally clear info about PTEs") Signed-off-by: Ryan Roberts <ryan.roberts@arm.com> --- fs/proc/task_mmu.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) -- 2.25.1