[PATCH v4] mm/gup_test: safely calculate GUP batch size

[PATCH v4] mm/gup_test: safely calculate GUP batch size

[Sarthak Sharma]

__gup_test_ioctl() calculates the end of a GUP batch using:

next = addr + nr * PAGE_SIZE;

If nr is too large, it can cause next to overflow and wrap around.
If it wraps, the next > end check is bypassed and a large value
of nr is passed to the GUP call, even though the pages array was
allocated according to gup->size. This can lead to out of bounds writes.

Also, when fewer than PAGE_SIZE bytes remain, the calculated batch
contains zero pages. The code still calls GUP functions with pages + i,
which can point past the allocated array.

Calculate nr by taking the minimum of the number of pages per call and
the pages remaining in the address range. Stop processing when no
pages remain and calculate the end of the batch using this bounded
value of nr.

Fixes: 64c349f4ae78 ("mm: add infrastructure for get_user_pages_fast() benchmarking")
Signed-off-by: Sarthak Sharma <sarthak.sharma@arm.com>
---
Sashiko reported the issue fixed by this patch while reviewing the patch
"mm/gup_test: report actual pinned bytes". That patch has been applied to
mm-new. Since the two fixes are independent, this revision contains
only the GUP batch size fix.

Changes in v4:
- Simplify batch clamping logic as suggested by Kiryl
- Don't call GUP functions for an empty batch
- Drop the pinned byte reporting patch since it has been applied to mm-new

v3: https://lore.kernel.org/all/20260901083452.115365-1-sarthak.sharma@arm.com/

 mm/gup_test.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/mm/gup_test.c b/mm/gup_test.c
index 185ba3bb8ed1..1b64e932c4c5 100644
--- a/mm/gup_test.c
+++ b/mm/gup_test.c
@@ -139,11 +139,12 @@ static int __gup_test_ioctl(unsigned int cmd,
 		if (nr != gup->nr_pages_per_call)
 			break;
 
+		nr = min_t(unsigned long, gup->nr_pages_per_call,
+			   (end - addr) / PAGE_SIZE);
+		if (!nr)
+			break;
+
 		next = addr + nr * PAGE_SIZE;
-		if (next > end) {
-			next = end;
-			nr = (next - addr) / PAGE_SIZE;
-		}
 
 		switch (cmd) {
 		case GUP_FAST_BENCHMARK:

base-commit: 178b3d97bf1f15f598ea7cc615a40c115e528e1c
-- 
2.53.0

Re: [PATCH v4] mm/gup_test: safely calculate GUP batch size

[Kiryl Shutsemau]

On Thu, Sep 03, 2026 at 05:20:33PM +0530, Sarthak Sharma wrote:
> __gup_test_ioctl() calculates the end of a GUP batch using:
> 
> next = addr + nr * PAGE_SIZE;
> 
> If nr is too large, it can cause next to overflow and wrap around.
> If it wraps, the next > end check is bypassed and a large value
> of nr is passed to the GUP call, even though the pages array was
> allocated according to gup->size. This can lead to out of bounds writes.
> 
> Also, when fewer than PAGE_SIZE bytes remain, the calculated batch
> contains zero pages. The code still calls GUP functions with pages + i,
> which can point past the allocated array.
> 
> Calculate nr by taking the minimum of the number of pages per call and
> the pages remaining in the address range. Stop processing when no
> pages remain and calculate the end of the batch using this bounded
> value of nr.
> 
> Fixes: 64c349f4ae78 ("mm: add infrastructure for get_user_pages_fast() benchmarking")
> Signed-off-by: Sarthak Sharma <sarthak.sharma@arm.com>

Reviewed-by: Kiryl Shutsemau (Meta) <kas@kernel.org>

On nit below.

> ---
> Sashiko reported the issue fixed by this patch while reviewing the patch
> "mm/gup_test: report actual pinned bytes". That patch has been applied to
> mm-new. Since the two fixes are independent, this revision contains
> only the GUP batch size fix.
> 
> Changes in v4:
> - Simplify batch clamping logic as suggested by Kiryl
> - Don't call GUP functions for an empty batch
> - Drop the pinned byte reporting patch since it has been applied to mm-new
> 
> v3: https://lore.kernel.org/all/20260901083452.115365-1-sarthak.sharma@arm.com/
> 
>  mm/gup_test.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/gup_test.c b/mm/gup_test.c
> index 185ba3bb8ed1..1b64e932c4c5 100644
> --- a/mm/gup_test.c
> +++ b/mm/gup_test.c
> @@ -139,11 +139,12 @@ static int __gup_test_ioctl(unsigned int cmd,
>  		if (nr != gup->nr_pages_per_call)
>  			break;
>  
> +		nr = min_t(unsigned long, gup->nr_pages_per_call,
> +			   (end - addr) / PAGE_SIZE);

I personally would rather have it in two statements, as I suggested
initially:

		nr = gup->nr_pages_per_call;
		nr = min_t(unsigned long, nr, (end - addr) / PAGE_SIZE);

It seems to be more readable to me.


> +		if (!nr)
> +			break;
> +
>  		next = addr + nr * PAGE_SIZE;
> -		if (next > end) {
> -			next = end;
> -			nr = (next - addr) / PAGE_SIZE;
> -		}
>  
>  		switch (cmd) {
>  		case GUP_FAST_BENCHMARK:
> 
> base-commit: 178b3d97bf1f15f598ea7cc615a40c115e528e1c
> -- 
> 2.53.0
> 

-- 
  Kiryl Shutsemau / Kirill A. Shutemov

Re: [PATCH v4] mm/gup_test: safely calculate GUP batch size

[Sarthak Sharma]

Hi Kiryl!

On 9/3/26 6:09 PM, Kiryl Shutsemau wrote:
> On Thu, Sep 03, 2026 at 05:20:33PM +0530, Sarthak Sharma wrote:
>> __gup_test_ioctl() calculates the end of a GUP batch using:
>>
>> next = addr + nr * PAGE_SIZE;
>>
>> If nr is too large, it can cause next to overflow and wrap around.
>> If it wraps, the next > end check is bypassed and a large value
>> of nr is passed to the GUP call, even though the pages array was
>> allocated according to gup->size. This can lead to out of bounds writes.
>>
>> Also, when fewer than PAGE_SIZE bytes remain, the calculated batch
>> contains zero pages. The code still calls GUP functions with pages + i,
>> which can point past the allocated array.
>>
>> Calculate nr by taking the minimum of the number of pages per call and
>> the pages remaining in the address range. Stop processing when no
>> pages remain and calculate the end of the batch using this bounded
>> value of nr.
>>
>> Fixes: 64c349f4ae78 ("mm: add infrastructure for get_user_pages_fast() benchmarking")
>> Signed-off-by: Sarthak Sharma <sarthak.sharma@arm.com>
> 
> Reviewed-by: Kiryl Shutsemau (Meta) <kas@kernel.org>

Thank you!

> 
> On nit below.
> 
>> ---
>> Sashiko reported the issue fixed by this patch while reviewing the patch
>> "mm/gup_test: report actual pinned bytes". That patch has been applied to
>> mm-new. Since the two fixes are independent, this revision contains
>> only the GUP batch size fix.
>>
>> Changes in v4:
>> - Simplify batch clamping logic as suggested by Kiryl
>> - Don't call GUP functions for an empty batch
>> - Drop the pinned byte reporting patch since it has been applied to mm-new
>>
>> v3: https://lore.kernel.org/all/20260901083452.115365-1-sarthak.sharma@arm.com/
>>
>>  mm/gup_test.c | 9 +++++----
>>  1 file changed, 5 insertions(+), 4 deletions(-)
>>
>> diff --git a/mm/gup_test.c b/mm/gup_test.c
>> index 185ba3bb8ed1..1b64e932c4c5 100644
>> --- a/mm/gup_test.c
>> +++ b/mm/gup_test.c
>> @@ -139,11 +139,12 @@ static int __gup_test_ioctl(unsigned int cmd,
>>  		if (nr != gup->nr_pages_per_call)
>>  			break;
>>  
>> +		nr = min_t(unsigned long, gup->nr_pages_per_call,
>> +			   (end - addr) / PAGE_SIZE);
> 
> I personally would rather have it in two statements, as I suggested
> initially:
> 
> 		nr = gup->nr_pages_per_call;
> 		nr = min_t(unsigned long, nr, (end - addr) / PAGE_SIZE);
> 
> It seems to be more readable to me.

Both forms are equivalent, and I would prefer to keep it in a single
line instead of having two statements to do the same thing. I don't
think I need to respin with this change unless you feel strongly about it :)
> 
> 
>> +		if (!nr)
>> +			break;
>> +
>>  		next = addr + nr * PAGE_SIZE;
>> -		if (next > end) {
>> -			next = end;
>> -			nr = (next - addr) / PAGE_SIZE;
>> -		}
>>  
>>  		switch (cmd) {
>>  		case GUP_FAST_BENCHMARK:
>>
>> base-commit: 178b3d97bf1f15f598ea7cc615a40c115e528e1c
>> -- 
>> 2.53.0
>>
> 

Re: [PATCH v4] mm/gup_test: safely calculate GUP batch size

[David Hildenbrand (Arm)]

On 9/3/26 13:50, Sarthak Sharma wrote:
> __gup_test_ioctl() calculates the end of a GUP batch using:
> 
> next = addr + nr * PAGE_SIZE;
> 
> If nr is too large, it can cause next to overflow and wrap around.
> If it wraps, the next > end check is bypassed and a large value
> of nr is passed to the GUP call, even though the pages array was
> allocated according to gup->size. This can lead to out of bounds writes.
> 
> Also, when fewer than PAGE_SIZE bytes remain, the calculated batch
> contains zero pages. The code still calls GUP functions with pages + i,
> which can point past the allocated array.
> 
> Calculate nr by taking the minimum of the number of pages per call and
> the pages remaining in the address range. Stop processing when no
> pages remain and calculate the end of the batch using this bounded
> value of nr.
> 
> Fixes: 64c349f4ae78 ("mm: add infrastructure for get_user_pages_fast() benchmarking")
> Signed-off-by: Sarthak Sharma <sarthak.sharma@arm.com>
> ---
> Sashiko reported the issue fixed by this patch while reviewing the patch
> "mm/gup_test: report actual pinned bytes". That patch has been applied to
> mm-new. Since the two fixes are independent, this revision contains
> only the GUP batch size fix.
> 
> Changes in v4:
> - Simplify batch clamping logic as suggested by Kiryl
> - Don't call GUP functions for an empty batch
> - Drop the pinned byte reporting patch since it has been applied to mm-new
> 
> v3: https://lore.kernel.org/all/20260901083452.115365-1-sarthak.sharma@arm.com/
> 
>  mm/gup_test.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/gup_test.c b/mm/gup_test.c
> index 185ba3bb8ed1..1b64e932c4c5 100644
> --- a/mm/gup_test.c
> +++ b/mm/gup_test.c
> @@ -139,11 +139,12 @@ static int __gup_test_ioctl(unsigned int cmd,
>  		if (nr != gup->nr_pages_per_call)
>  			break;
>  
> +		nr = min_t(unsigned long, gup->nr_pages_per_call,
> +			   (end - addr) / PAGE_SIZE);
> +		if (!nr)
> +			break;

Can !nr actually happen?

-- 
Cheers,

David

Re: [PATCH v4] mm/gup_test: safely calculate GUP batch size

[Sarthak Sharma]

Hi David!

On 9/7/26 8:20 PM, David Hildenbrand (Arm) wrote:
> On 9/3/26 13:50, Sarthak Sharma wrote:
>> __gup_test_ioctl() calculates the end of a GUP batch using:
>>
>> next = addr + nr * PAGE_SIZE;
>>
>> If nr is too large, it can cause next to overflow and wrap around.
>> If it wraps, the next > end check is bypassed and a large value
>> of nr is passed to the GUP call, even though the pages array was
>> allocated according to gup->size. This can lead to out of bounds writes.
>>
>> Also, when fewer than PAGE_SIZE bytes remain, the calculated batch
>> contains zero pages. The code still calls GUP functions with pages + i,
>> which can point past the allocated array.
>>
>> Calculate nr by taking the minimum of the number of pages per call and
>> the pages remaining in the address range. Stop processing when no
>> pages remain and calculate the end of the batch using this bounded
>> value of nr.
>>
>> Fixes: 64c349f4ae78 ("mm: add infrastructure for get_user_pages_fast() benchmarking")
>> Signed-off-by: Sarthak Sharma <sarthak.sharma@arm.com>
>> ---
>> Sashiko reported the issue fixed by this patch while reviewing the patch
>> "mm/gup_test: report actual pinned bytes". That patch has been applied to
>> mm-new. Since the two fixes are independent, this revision contains
>> only the GUP batch size fix.
>>
>> Changes in v4:
>> - Simplify batch clamping logic as suggested by Kiryl
>> - Don't call GUP functions for an empty batch
>> - Drop the pinned byte reporting patch since it has been applied to mm-new
>>
>> v3: https://lore.kernel.org/all/20260901083452.115365-1-sarthak.sharma@arm.com/
>>
>>  mm/gup_test.c | 9 +++++----
>>  1 file changed, 5 insertions(+), 4 deletions(-)
>>
>> diff --git a/mm/gup_test.c b/mm/gup_test.c
>> index 185ba3bb8ed1..1b64e932c4c5 100644
>> --- a/mm/gup_test.c
>> +++ b/mm/gup_test.c
>> @@ -139,11 +139,12 @@ static int __gup_test_ioctl(unsigned int cmd,
>>  		if (nr != gup->nr_pages_per_call)
>>  			break;
>>  
>> +		nr = min_t(unsigned long, gup->nr_pages_per_call,
>> +			   (end - addr) / PAGE_SIZE);
>> +		if (!nr)
>> +			break;
> 
> Can !nr actually happen?

Yes, it is possible in 2 cases:

1. We calculate end as gup->addr + gup->size. So, if gup->size is not
page aligned, then at some point, we can have (end - addr) < PAGE_SIZE
and (end != addr). Then (end - addr) / PAGESIZE will be 0. Right now all
callers seem to provide a page aligned gup->size, but any userspace
program which can open /sys/kernel/debug/gup_test can provide a non page
aligned gup->size.

2. If the userspace supplies nr_pages_per_call = 0 (we never validate
nr_pages_per_call so it is possible).

Sorry for the delay, your email didn't reach my inbox due to some reason :)

Re: [PATCH v4] mm/gup_test: safely calculate GUP batch size

[David Hildenbrand (Arm)]

On 9/11/26 14:13, Sarthak Sharma wrote:
> Hi David!
> 
> On 9/7/26 8:20 PM, David Hildenbrand (Arm) wrote:
>> On 9/3/26 13:50, Sarthak Sharma wrote:
>>> __gup_test_ioctl() calculates the end of a GUP batch using:
>>>
>>> next = addr + nr * PAGE_SIZE;
>>>
>>> If nr is too large, it can cause next to overflow and wrap around.
>>> If it wraps, the next > end check is bypassed and a large value
>>> of nr is passed to the GUP call, even though the pages array was
>>> allocated according to gup->size. This can lead to out of bounds writes.
>>>
>>> Also, when fewer than PAGE_SIZE bytes remain, the calculated batch
>>> contains zero pages. The code still calls GUP functions with pages + i,
>>> which can point past the allocated array.
>>>
>>> Calculate nr by taking the minimum of the number of pages per call and
>>> the pages remaining in the address range. Stop processing when no
>>> pages remain and calculate the end of the batch using this bounded
>>> value of nr.
>>>
>>> Fixes: 64c349f4ae78 ("mm: add infrastructure for get_user_pages_fast() benchmarking")
>>> Signed-off-by: Sarthak Sharma <sarthak.sharma@arm.com>
>>> ---
>>> Sashiko reported the issue fixed by this patch while reviewing the patch
>>> "mm/gup_test: report actual pinned bytes". That patch has been applied to
>>> mm-new. Since the two fixes are independent, this revision contains
>>> only the GUP batch size fix.
>>>
>>> Changes in v4:
>>> - Simplify batch clamping logic as suggested by Kiryl
>>> - Don't call GUP functions for an empty batch
>>> - Drop the pinned byte reporting patch since it has been applied to mm-new
>>>
>>> v3: https://lore.kernel.org/all/20260901083452.115365-1-sarthak.sharma@arm.com/
>>>
>>>  mm/gup_test.c | 9 +++++----
>>>  1 file changed, 5 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/mm/gup_test.c b/mm/gup_test.c
>>> index 185ba3bb8ed1..1b64e932c4c5 100644
>>> --- a/mm/gup_test.c
>>> +++ b/mm/gup_test.c
>>> @@ -139,11 +139,12 @@ static int __gup_test_ioctl(unsigned int cmd,
>>>  		if (nr != gup->nr_pages_per_call)
>>>  			break;
>>>  
>>> +		nr = min_t(unsigned long, gup->nr_pages_per_call,
>>> +			   (end - addr) / PAGE_SIZE);
>>> +		if (!nr)
>>> +			break;
>>
>> Can !nr actually happen?
> 
> Yes, it is possible in 2 cases:
> 
> 1. We calculate end as gup->addr + gup->size. So, if gup->size is not
> page aligned, then at some point, we can have (end - addr) < PAGE_SIZE
> and (end != addr). Then (end - addr) / PAGESIZE will be 0. Right now all
> callers seem to provide a page aligned gup->size, but any userspace
> program which can open /sys/kernel/debug/gup_test can provide a non page
> aligned gup->size.

Note that this is a pure testing interface. So we don't care about weird user
space doing stupid things.

Rather reject unaligned gup->size early in __gup_test_ioctl().

> 
> 2. If the userspace supplies nr_pages_per_call = 0 (we never validate
> nr_pages_per_call so it is possible).
> 

Same here, user space won't do that. If you want to be sure, rather reject
exactly that early in __gup_test_ioctl().

Something like

diff --git a/mm/gup_test.c b/mm/gup_test.c
index 185ba3bb8ed10..9e3e1df8dd526 100644
--- a/mm/gup_test.c
+++ b/mm/gup_test.c
@@ -116,7 +116,8 @@ static int __gup_test_ioctl(unsigned int cmd,
        bool needs_mmap_lock =
                cmd != GUP_FAST_BENCHMARK && cmd != PIN_FAST_BENCHMARK;

-       if (gup->addr > ULONG_MAX || gup->size > ULONG_MAX)
+       if (gup->addr > ULONG_MAX || gup->size > ULONG_MAX || !gup->size ||
+           !PAGE_ALIGNED(gup->size))
                return -EINVAL;
        if (check_add_overflow((unsigned long)gup->addr,
                               (unsigned long)gup->size, &end))

> Sorry for the delay, your email didn't reach my inbox due to some reason :)

No worries.

-- 
Cheers,

David