Commit c865f98d authored by Linus Torvalds's avatar Linus Torvalds Committed by Sasha Levin

mm: remove gup_flags FOLL_WRITE games from __get_user_pages()

[ Upstream commit 19be0eaf ]

This is an ancient bug that was actually attempted to be fixed once
(badly) by me eleven years ago in commit 4ceb5db9 ("Fix
get_user_pages() race for write access") but that was then undone due to
problems on s390 by commit f33ea7f4 ("fix get_user_pages bug").

In the meantime, the s390 situation has long been fixed, and we can now
fix it by checking the pte_dirty() bit properly (and do it better).  The
s390 dirty bit was implemented in abf09bed ("s390/mm: implement
software dirty bits") which made it into v3.9.  Earlier kernels will
have to look at the page state itself.

Also, the VM has become more scalable, and what used a purely
theoretical race back then has become easier to trigger.

To fix it, we introduce a new internal FOLL_COW flag to mark the "yes,
we already did a COW" rather than play racy games with FOLL_WRITE that
is very fundamental, and then use the pte dirty flag to validate that
the FOLL_COW flag is still valid.
Reported-and-tested-by: default avatarPhil "not Paul" Oester <kernel@linuxace.com>
Acked-by: default avatarHugh Dickins <hughd@google.com>
Reviewed-by: default avatarMichal Hocko <mhocko@suse.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Willy Tarreau <w@1wt.eu>
Cc: Nick Piggin <npiggin@gmail.com>
Cc: Greg Thelen <gthelen@google.com>
Cc: stable@vger.kernel.org
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: default avatarSasha Levin <alexander.levin@verizon.com>
parent 4edf04a3
...@@ -2064,6 +2064,7 @@ static inline struct page *follow_page(struct vm_area_struct *vma, ...@@ -2064,6 +2064,7 @@ static inline struct page *follow_page(struct vm_area_struct *vma,
#define FOLL_NUMA 0x200 /* force NUMA hinting page fault */ #define FOLL_NUMA 0x200 /* force NUMA hinting page fault */
#define FOLL_MIGRATION 0x400 /* wait for page to replace migration entry */ #define FOLL_MIGRATION 0x400 /* wait for page to replace migration entry */
#define FOLL_TRIED 0x800 /* a retry, previous pass started an IO */ #define FOLL_TRIED 0x800 /* a retry, previous pass started an IO */
#define FOLL_COW 0x4000 /* internal GUP flag */
typedef int (*pte_fn_t)(pte_t *pte, pgtable_t token, unsigned long addr, typedef int (*pte_fn_t)(pte_t *pte, pgtable_t token, unsigned long addr,
void *data); void *data);
......
...@@ -32,6 +32,16 @@ static struct page *no_page_table(struct vm_area_struct *vma, ...@@ -32,6 +32,16 @@ static struct page *no_page_table(struct vm_area_struct *vma,
return NULL; return NULL;
} }
/*
* FOLL_FORCE can write to even unwritable pte's, but only
* after we've gone through a COW cycle and they are dirty.
*/
static inline bool can_follow_write_pte(pte_t pte, unsigned int flags)
{
return pte_write(pte) ||
((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_dirty(pte));
}
static struct page *follow_page_pte(struct vm_area_struct *vma, static struct page *follow_page_pte(struct vm_area_struct *vma,
unsigned long address, pmd_t *pmd, unsigned int flags) unsigned long address, pmd_t *pmd, unsigned int flags)
{ {
...@@ -66,7 +76,7 @@ static struct page *follow_page_pte(struct vm_area_struct *vma, ...@@ -66,7 +76,7 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
} }
if ((flags & FOLL_NUMA) && pte_protnone(pte)) if ((flags & FOLL_NUMA) && pte_protnone(pte))
goto no_page; goto no_page;
if ((flags & FOLL_WRITE) && !pte_write(pte)) { if ((flags & FOLL_WRITE) && !can_follow_write_pte(pte, flags)) {
pte_unmap_unlock(ptep, ptl); pte_unmap_unlock(ptep, ptl);
return NULL; return NULL;
} }
...@@ -315,7 +325,7 @@ static int faultin_page(struct task_struct *tsk, struct vm_area_struct *vma, ...@@ -315,7 +325,7 @@ static int faultin_page(struct task_struct *tsk, struct vm_area_struct *vma,
* reCOWed by userspace write). * reCOWed by userspace write).
*/ */
if ((ret & VM_FAULT_WRITE) && !(vma->vm_flags & VM_WRITE)) if ((ret & VM_FAULT_WRITE) && !(vma->vm_flags & VM_WRITE))
*flags &= ~FOLL_WRITE; *flags |= FOLL_COW;
return 0; return 0;
} }
......
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