Re: [PATCH 1/1] gup: document and work around "COW can break either way" issue

[Jan Kara <jack@suse.cz>]

On Mon 11-10-21 18:52:44, Suren Baghdasaryan wrote:
> From: Linus Torvalds <torvalds@linux-foundation.org>
> 
> From: Linus Torvalds <torvalds@linux-foundation.org>
> 
> commit 17839856fd588f4ab6b789f482ed3ffd7c403e1f upstream.
> 
> Doing a "get_user_pages()" on a copy-on-write page for reading can be
> ambiguous: the page can be COW'ed at any time afterwards, and the
> direction of a COW event isn't defined.
> 
> Yes, whoever writes to it will generally do the COW, but if the thread
> that did the get_user_pages() unmapped the page before the write (and
> that could happen due to memory pressure in addition to any outright
> action), the writer could also just take over the old page instead.
> 
> End result: the get_user_pages() call might result in a page pointer
> that is no longer associated with the original VM, and is associated
> with - and controlled by - another VM having taken it over instead.
> 
> So when doing a get_user_pages() on a COW mapping, the only really safe
> thing to do would be to break the COW when getting the page, even when
> only getting it for reading.
> 
> At the same time, some users simply don't even care.
> 
> For example, the perf code wants to look up the page not because it
> cares about the page, but because the code simply wants to look up the
> physical address of the access for informational purposes, and doesn't
> really care about races when a page might be unmapped and remapped
> elsewhere.
> 
> This adds logic to force a COW event by setting FOLL_WRITE on any
> copy-on-write mapping when FOLL_GET (or FOLL_PIN) is used to get a page
> pointer as a result.
> 
> The current semantics end up being:
> 
>  - __get_user_pages_fast(): no change. If you don't ask for a write,
>    you won't break COW. You'd better know what you're doing.
> 
>  - get_user_pages_fast(): the fast-case "look it up in the page tables
>    without anything getting mmap_sem" now refuses to follow a read-only
>    page, since it might need COW breaking.  Which happens in the slow
>    path - the fast path doesn't know if the memory might be COW or not.
> 
>  - get_user_pages() (including the slow-path fallback for gup_fast()):
>    for a COW mapping, turn on FOLL_WRITE for FOLL_GET/FOLL_PIN, with
>    very similar semantics to FOLL_FORCE.
> 
> If it turns out that we want finer granularity (ie "only break COW when
> it might actually matter" - things like the zero page are special and
> don't need to be broken) we might need to push these semantics deeper
> into the lookup fault path.  So if people care enough, it's possible
> that we might end up adding a new internal FOLL_BREAK_COW flag to go
> with the internal FOLL_COW flag we already have for tracking "I had a
> COW".
> 
> Alternatively, if it turns out that different callers might want to
> explicitly control the forced COW break behavior, we might even want to
> make such a flag visible to the users of get_user_pages() instead of
> using the above default semantics.
> 
> But for now, this is mostly commentary on the issue (this commit message
> being a lot bigger than the patch, and that patch in turn is almost all
> comments), with that minimal "enable COW breaking early" logic using the
> existing FOLL_WRITE behavior.
> 
> [ It might be worth noting that we've always had this ambiguity, and it
>   could arguably be seen as a user-space issue.
> 
>   You only get private COW mappings that could break either way in
>   situations where user space is doing cooperative things (ie fork()
>   before an execve() etc), but it _is_ surprising and very subtle, and
>   fork() is supposed to give you independent address spaces.
> 
>   So let's treat this as a kernel issue and make the semantics of
>   get_user_pages() easier to understand. Note that obviously a true
>   shared mapping will still get a page that can change under us, so this
>   does _not_ mean that get_user_pages() somehow returns any "stable"
>   page ]
> 
> [surenb: backport notes
>         Since gup_pgd_range does not exist, made appropriate changes on
>         the the gup_huge_pgd, gup_huge_pd and gup_pud_range calls instead.
> 	Replaced (gup_flags | FOLL_WRITE) with write=1 in gup_huge_pgd,
>         gup_huge_pd and gup_pud_range.
> 	Removed FOLL_PIN usage in should_force_cow_break since it's missing in
> 	the earlier kernels.]

I'd be really careful with backporting this to stable. There was a lot of
userspace breakage caused by this change if I remember right which needed
to be fixed up later. There is a nice summary at
https://lwn.net/Articles/849638/ and https://lwn.net/Articles/849876/ and
some problems are still being found...

								Honza

> 
> Reported-by: Jann Horn <jannh@google.com>
> Tested-by: Christoph Hellwig <hch@lst.de>
> Acked-by: Oleg Nesterov <oleg@redhat.com>
> Acked-by: Kirill Shutemov <kirill@shutemov.name>
> Acked-by: Jan Kara <jack@suse.cz>
> Cc: Andrea Arcangeli <aarcange@redhat.com>
> Cc: Matthew Wilcox <willy@infradead.org>
> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
> [surenb: backport to 4.4 kernel]
> Cc: stable@vger.kernel.org # 4.4.x
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> ---
>  mm/gup.c         | 48 ++++++++++++++++++++++++++++++++++++++++--------
>  mm/huge_memory.c |  7 +++----
>  2 files changed, 43 insertions(+), 12 deletions(-)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index 4c5857889e9d..c80cdc408228 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -59,13 +59,22 @@ static int follow_pfn_pte(struct vm_area_struct *vma, unsigned long address,
>  }
>  
>  /*
> - * FOLL_FORCE can write to even unwritable pte's, but only
> - * after we've gone through a COW cycle and they are dirty.
> + * FOLL_FORCE or a forced COW break can write even to 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));
> +	return pte_write(pte) || ((flags & FOLL_COW) && pte_dirty(pte));
> +}
> +
> +/*
> + * A (separate) COW fault might break the page the other way and
> + * get_user_pages() would return the page from what is now the wrong
> + * VM. So we need to force a COW break at GUP time even for reads.
> + */
> +static inline bool should_force_cow_break(struct vm_area_struct *vma, unsigned int flags)
> +{
> +	return is_cow_mapping(vma->vm_flags) && (flags & FOLL_GET);
>  }
>  
>  static struct page *follow_page_pte(struct vm_area_struct *vma,
> @@ -509,12 +518,18 @@ long __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
>  			if (!vma || check_vma_flags(vma, gup_flags))
>  				return i ? : -EFAULT;
>  			if (is_vm_hugetlb_page(vma)) {
> +				if (should_force_cow_break(vma, foll_flags))
> +					foll_flags |= FOLL_WRITE;
>  				i = follow_hugetlb_page(mm, vma, pages, vmas,
>  						&start, &nr_pages, i,
> -						gup_flags);
> +						foll_flags);
>  				continue;
>  			}
>  		}
> +
> +		if (should_force_cow_break(vma, foll_flags))
> +			foll_flags |= FOLL_WRITE;
> +
>  retry:
>  		/*
>  		 * If we have a pending SIGKILL, don't keep faulting pages and
> @@ -1346,6 +1361,10 @@ static int gup_pud_range(pgd_t pgd, unsigned long addr, unsigned long end,
>  /*
>   * Like get_user_pages_fast() except it's IRQ-safe in that it won't fall back to
>   * the regular GUP. It will only return non-negative values.
> + *
> + * Careful, careful! COW breaking can go either way, so a non-write
> + * access can get ambiguous page results. If you call this function without
> + * 'write' set, you'd better be sure that you're ok with that ambiguity.
>   */
>  int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
>  			  struct page **pages)
> @@ -1375,6 +1394,12 @@ int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
>  	 *
>  	 * We do not adopt an rcu_read_lock(.) here as we also want to
>  	 * block IPIs that come from THPs splitting.
> +	 *
> +	 * NOTE! We allow read-only gup_fast() here, but you'd better be
> +	 * careful about possible COW pages. You'll get _a_ COW page, but
> +	 * not necessarily the one you intended to get depending on what
> +	 * COW event happens after this. COW may break the page copy in a
> +	 * random direction.
>  	 */
>  
>  	local_irq_save(flags);
> @@ -1385,15 +1410,22 @@ int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
>  		next = pgd_addr_end(addr, end);
>  		if (pgd_none(pgd))
>  			break;
> +		/*
> +		 * The FAST_GUP case requires FOLL_WRITE even for pure reads,
> +		 * because get_user_pages() may need to cause an early COW in
> +		 * order to avoid confusing the normal COW routines. So only
> +		 * targets that are already writable are safe to do by just
> +		 * looking at the page tables.
> +		 */
>  		if (unlikely(pgd_huge(pgd))) {
> -			if (!gup_huge_pgd(pgd, pgdp, addr, next, write,
> +			if (!gup_huge_pgd(pgd, pgdp, addr, next, 1,
>  					  pages, &nr))
>  				break;
>  		} else if (unlikely(is_hugepd(__hugepd(pgd_val(pgd))))) {
>  			if (!gup_huge_pd(__hugepd(pgd_val(pgd)), addr,
> -					 PGDIR_SHIFT, next, write, pages, &nr))
> +					 PGDIR_SHIFT, next, 1, pages, &nr))
>  				break;
> -		} else if (!gup_pud_range(pgd, addr, next, write, pages, &nr))
> +		} else if (!gup_pud_range(pgd, addr, next, 1, pages, &nr))
>  			break;
>  	} while (pgdp++, addr = next, addr != end);
>  	local_irq_restore(flags);
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 6404e4fcb4ed..fae45c56e2ee 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1268,13 +1268,12 @@ out_unlock:
>  }
>  
>  /*
> - * FOLL_FORCE can write to even unwritable pmd's, but only
> - * after we've gone through a COW cycle and they are dirty.
> + * FOLL_FORCE or a forced COW break can write even to unwritable pmd's,
> + * but only after we've gone through a COW cycle and they are dirty.
>   */
>  static inline bool can_follow_write_pmd(pmd_t pmd, unsigned int flags)
>  {
> -	return pmd_write(pmd) ||
> -	       ((flags & FOLL_FORCE) && (flags & FOLL_COW) && pmd_dirty(pmd));
> +	return pmd_write(pmd) || ((flags & FOLL_COW) && pmd_dirty(pmd));
>  }
>  
>  struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
> -- 
> 2.33.0.882.g93a45727a2-goog
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR