[PATCH] iomap: Fix overflow in iomap_page_mkwrite

[PATCH] iomap: Fix overflow in iomap_page_mkwrite

[Andreas Gruenbacher]

On architectures where ssize_t is wider than pgoff_t, the expression
((page->index + 1) << PAGE_SHIFT) can overflow.  Rewrite to use the page
offset, which we already compute here anyway.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
---
 fs/iomap/buffered-io.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
index e25901ae3ff4..a30ea7ecb790 100644
--- a/fs/iomap/buffered-io.c
+++ b/fs/iomap/buffered-io.c
@@ -1040,20 +1040,19 @@ vm_fault_t iomap_page_mkwrite(struct vm_fault *vmf, const struct iomap_ops *ops)
 
 	lock_page(page);
 	size = i_size_read(inode);
-	if ((page->mapping != inode->i_mapping) ||
-	    (page_offset(page) > size)) {
+	offset = page_offset(page);
+	if (page->mapping != inode->i_mapping || offset > size) {
 		/* We overload EFAULT to mean page got truncated */
 		ret = -EFAULT;
 		goto out_unlock;
 	}
 
 	/* page is wholly or partially inside EOF */
-	if (((page->index + 1) << PAGE_SHIFT) > size)
+	if (offset > size - PAGE_SIZE)
 		length = offset_in_page(size);
 	else
 		length = PAGE_SIZE;
 
-	offset = page_offset(page);
 	while (length > 0) {
 		ret = iomap_apply(inode, offset, length,
 				IOMAP_WRITE | IOMAP_FAULT, ops, page,
-- 
2.20.1

Re: [PATCH] iomap: Fix overflow in iomap_page_mkwrite

[Darrick J. Wong]

On Wed, Nov 06, 2019 at 08:04:00PM +0100, Andreas Gruenbacher wrote:
> On architectures where ssize_t is wider than pgoff_t, the expression

ssize_t?  But you're changing @offset, which is loff_t.   I'm confused.

Also, which architectures are you talking about here?

--D

> ((page->index + 1) << PAGE_SHIFT) can overflow.  Rewrite to use the page
> offset, which we already compute here anyway.
> 
> Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
> ---
>  fs/iomap/buffered-io.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index e25901ae3ff4..a30ea7ecb790 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -1040,20 +1040,19 @@ vm_fault_t iomap_page_mkwrite(struct vm_fault *vmf, const struct iomap_ops *ops)
>  
>  	lock_page(page);
>  	size = i_size_read(inode);
> -	if ((page->mapping != inode->i_mapping) ||
> -	    (page_offset(page) > size)) {
> +	offset = page_offset(page);
> +	if (page->mapping != inode->i_mapping || offset > size) {
>  		/* We overload EFAULT to mean page got truncated */
>  		ret = -EFAULT;
>  		goto out_unlock;
>  	}
>  
>  	/* page is wholly or partially inside EOF */
> -	if (((page->index + 1) << PAGE_SHIFT) > size)
> +	if (offset > size - PAGE_SIZE)
>  		length = offset_in_page(size);
>  	else
>  		length = PAGE_SIZE;
>  
> -	offset = page_offset(page);
>  	while (length > 0) {
>  		ret = iomap_apply(inode, offset, length,
>  				IOMAP_WRITE | IOMAP_FAULT, ops, page,
> -- 
> 2.20.1
> 

Re: [PATCH] iomap: Fix overflow in iomap_page_mkwrite

[Andreas Gruenbacher]

On Wed, Nov 6, 2019 at 8:17 PM Darrick J. Wong <darrick.wong@oracle.com> wrote:
> On Wed, Nov 06, 2019 at 08:04:00PM +0100, Andreas Gruenbacher wrote:
> > On architectures where ssize_t is wider than pgoff_t, the expression
>
> ssize_t?  But you're changing @offset, which is loff_t.   I'm confused.

Oops, should have been loff_t instead of ssize_t.

> Also, which architectures are you talking about here?

From the kernel headers:

#define pgoff_t unsigned long
typedef long long __kernel_loff_t;
typedef __kernel_loff_t loff_t;

So for example, sizeof(loff_t) > sizeof(pgoff_t) on x86.

Thanks,
Andreas

Re: [PATCH] iomap: Fix overflow in iomap_page_mkwrite

[Christoph Hellwig]

On Wed, Nov 06, 2019 at 08:04:00PM +0100, Andreas Gruenbacher wrote:
> On architectures where ssize_t is wider than pgoff_t, the expression
> ((page->index + 1) << PAGE_SHIFT) can overflow.  Rewrite to use the page
> offset, which we already compute here anyway.
> 
> Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>

Looks good except for the ssize_t vs loff_t in the changelog:

Reviewed-by: Christoph Hellwig <hch@lst.de>

Re: [PATCH] iomap: Fix overflow in iomap_page_mkwrite

[Darrick J. Wong]

On Wed, Nov 06, 2019 at 08:34:26PM +0100, Andreas Gruenbacher wrote:
> On Wed, Nov 6, 2019 at 8:17 PM Darrick J. Wong <darrick.wong@oracle.com> wrote:
> > On Wed, Nov 06, 2019 at 08:04:00PM +0100, Andreas Gruenbacher wrote:
> > > On architectures where ssize_t is wider than pgoff_t, the expression
> >
> > ssize_t?  But you're changing @offset, which is loff_t.   I'm confused.
> 
> Oops, should have been loff_t instead of ssize_t.

I'll fix it on commit.

> > Also, which architectures are you talking about here?
> 
> From the kernel headers:
> 
> #define pgoff_t unsigned long
> typedef long long __kernel_loff_t;
> typedef __kernel_loff_t loff_t;
> 
> So for example, sizeof(loff_t) > sizeof(pgoff_t) on x86.

Ok, that's what I thought.

Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>

--D

> Thanks,
> Andreas
> 

Re: [PATCH] iomap: Fix overflow in iomap_page_mkwrite

[Andreas Gruenbacher]

On Thu, Nov 7, 2019 at 4:37 PM Darrick J. Wong <darrick.wong@oracle.com> wrote:
> I'll fix it on commit.

Thanks.

So now the one remaining issue I have with those two functions is why
we check for (offset > size) instead of (offset >= size) in

  if (page->mapping != inode->i_mapping || offset > size)

When (offset == size), we're clearly outside the page, and so we should fail?

Thanks,
Andreas