2021-11-16 17:44:30

by Christophe Leroy

Subject: [PATCH] gup: Avoid multiple user access locking/unlocking in fault_in_{read/write}able

fault_in_readable() and fault_in_writeable() perform __get_user()
and __put_user() in a loop, implying multiple user access
locking/unlocking.

To avoid that, use user access blocks.

Cc: Andreas Gruenbacher <[email protected]>
Signed-off-by: Christophe Leroy <[email protected]>
---
mm/gup.c | 18 ++++++++++--------
1 file changed, 10 insertions(+), 8 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index 2c51e9748a6a..be2a41feec7d 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1672,21 +1672,22 @@ size_t fault_in_writeable(char __user *uaddr, size_t size)

if (unlikely(size == 0))
return 0;
+ if (!user_write_access_begin(uaddr, size))
+ return size;
if (!PAGE_ALIGNED(uaddr)) {
- if (unlikely(__put_user(0, uaddr) != 0))
- return size;
+ unsafe_put_user(0, uaddr, out);
uaddr = (char __user *)PAGE_ALIGN((unsigned long)uaddr);
}
end = (char __user *)PAGE_ALIGN((unsigned long)start + size);
if (unlikely(end < start))
end = NULL;
while (uaddr != end) {
- if (unlikely(__put_user(0, uaddr) != 0))
- goto out;
+ unsafe_put_user(0, uaddr, out);
uaddr += PAGE_SIZE;
}

out:
+ user_write_access_end();
if (size > uaddr - start)
return size - (uaddr - start);
return 0;
@@ -1771,21 +1772,22 @@ size_t fault_in_readable(const char __user *uaddr, size_t size)

if (unlikely(size == 0))
return 0;
+ if (!user_read_access_begin(uaddr, size))
+ return size;
if (!PAGE_ALIGNED(uaddr)) {
- if (unlikely(__get_user(c, uaddr) != 0))
- return size;
+ unsafe_get_user(c, uaddr, out);
uaddr = (const char __user *)PAGE_ALIGN((unsigned long)uaddr);
}
end = (const char __user *)PAGE_ALIGN((unsigned long)start + size);
if (unlikely(end < start))
end = NULL;
while (uaddr != end) {
- if (unlikely(__get_user(c, uaddr) != 0))
- goto out;
+ unsafe_get_user(c, uaddr, out);
uaddr += PAGE_SIZE;
}

out:
+ user_read_access_end();
(void)c;
if (size > uaddr - start)
return size - (uaddr - start);
--
2.31.1

2021-11-17 09:28:15

by Andreas Gruenbacher

Subject: Re: [PATCH] gup: Avoid multiple user access locking/unlocking in fault_in_{read/write}able

On Tue, Nov 16, 2021 at 6:50 PM Christophe Leroy
<[email protected]> wrote:
> fault_in_readable() and fault_in_writeable() perform __get_user()
> and __put_user() in a loop, implying multiple user access
> locking/unlocking.
>
> To avoid that, use user access blocks.
>
> Cc: Andreas Gruenbacher <[email protected]>
> Signed-off-by: Christophe Leroy <[email protected]>

Reviewed-by: Andreas Gruenbacher <[email protected]>

> ---
> mm/gup.c | 18 ++++++++++--------
> 1 file changed, 10 insertions(+), 8 deletions(-)
>
> diff --git a/mm/gup.c b/mm/gup.c
> index 2c51e9748a6a..be2a41feec7d 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1672,21 +1672,22 @@ size_t fault_in_writeable(char __user *uaddr, size_t size)
>
> if (unlikely(size == 0))
> return 0;
> + if (!user_write_access_begin(uaddr, size))
> + return size;
> if (!PAGE_ALIGNED(uaddr)) {
> - if (unlikely(__put_user(0, uaddr) != 0))
> - return size;
> + unsafe_put_user(0, uaddr, out);
> uaddr = (char __user *)PAGE_ALIGN((unsigned long)uaddr);
> }
> end = (char __user *)PAGE_ALIGN((unsigned long)start + size);
> if (unlikely(end < start))
> end = NULL;
> while (uaddr != end) {
> - if (unlikely(__put_user(0, uaddr) != 0))
> - goto out;
> + unsafe_put_user(0, uaddr, out);
> uaddr += PAGE_SIZE;
> }
>
> out:
> + user_write_access_end();
> if (size > uaddr - start)
> return size - (uaddr - start);
> return 0;
> @@ -1771,21 +1772,22 @@ size_t fault_in_readable(const char __user *uaddr, size_t size)
>
> if (unlikely(size == 0))
> return 0;
> + if (!user_read_access_begin(uaddr, size))
> + return size;
> if (!PAGE_ALIGNED(uaddr)) {
> - if (unlikely(__get_user(c, uaddr) != 0))
> - return size;
> + unsafe_get_user(c, uaddr, out);
> uaddr = (const char __user *)PAGE_ALIGN((unsigned long)uaddr);
> }
> end = (const char __user *)PAGE_ALIGN((unsigned long)start + size);
> if (unlikely(end < start))
> end = NULL;
> while (uaddr != end) {
> - if (unlikely(__get_user(c, uaddr) != 0))
> - goto out;
> + unsafe_get_user(c, uaddr, out);
> uaddr += PAGE_SIZE;
> }
>
> out:
> + user_read_access_end();
> (void)c;
> if (size > uaddr - start)
> return size - (uaddr - start);
> --
> 2.31.1
>