In __get_user_pages(), it will traverse page table and take a reference
to the page the given user address corresponds to if GUP_GET or GUP_PIN
is set. However, it's not supported both GUP_GET and GUP_PIN are set.
Even though this check need be done, it should be done earlier, but not
doing it till entering into follow_page_pte() and failed.

Furthermore, this checking has been done in is_valid_gup_args() and all
external users of __get_user_pages() will call is_valid_gup_args() to
catch the illegal setting. We don't need to worry about internal users
of __get_user_pages() because the gup_flags are set by MM code correctly.

Here remove the checking in follow_page_pte(), and add VM_WARN_ON_ONCE()
to catch the possible exceptional setting just in case.

And also change the VM_BUG_ON to VM_WARN_ON_ONCE() for checking
(!!pages != !!(gup_flags & (FOLL_GET | FOLL_PIN))) because the checking
has been done in is_valid_gup_args() for external users of __get_user_pages().

Signed-off-by: Baoquan He <bhe@redhat.com>
---
 mm/gup.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index 67a7de9e4f80..5b3ac5a867a3 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -844,11 +844,6 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
 	pte_t *ptep, pte;
 	int ret;
 
-	/* FOLL_GET and FOLL_PIN are mutually exclusive. */
-	if (WARN_ON_ONCE((flags & (FOLL_PIN | FOLL_GET)) ==
-			 (FOLL_PIN | FOLL_GET)))
-		return ERR_PTR(-EINVAL);
-
 	ptep = pte_offset_map_lock(mm, pmd, address, &ptl);
 	if (!ptep)
 		return no_page_table(vma, flags, address);
@@ -1432,7 +1427,11 @@ static long __get_user_pages(struct mm_struct *mm,
 
 	start = untagged_addr_remote(mm, start);
 
-	VM_BUG_ON(!!pages != !!(gup_flags & (FOLL_GET | FOLL_PIN)));
+	VM_WARN_ON_ONCE(!!pages != !!(gup_flags & (FOLL_GET | FOLL_PIN)));
+
+	/* FOLL_GET and FOLL_PIN are mutually exclusive. */
+	VM_WARN_ON_ONCE((gup_flags & (FOLL_PIN | FOLL_GET)) ==
+			(FOLL_PIN | FOLL_GET));
 
 	do {
 		struct page *page;
-- 
2.41.0

On Mon, Apr 07, 2025 at 11:03:05AM +0800, Baoquan He wrote:
> In __get_user_pages(), it will traverse page table and take a reference
> to the page the given user address corresponds to if GUP_GET or GUP_PIN
> is set. However, it's not supported both GUP_GET and GUP_PIN are set.
> Even though this check need be done, it should be done earlier, but not
> doing it till entering into follow_page_pte() and failed.
> 
> Furthermore, this checking has been done in is_valid_gup_args() and all
> external users of __get_user_pages() will call is_valid_gup_args() to
> catch the illegal setting. We don't need to worry about internal users
> of __get_user_pages() because the gup_flags are set by MM code correctly.
> 
> Here remove the checking in follow_page_pte(), and add VM_WARN_ON_ONCE()
> to catch the possible exceptional setting just in case.
> 
> And also change the VM_BUG_ON to VM_WARN_ON_ONCE() for checking
> (!!pages != !!(gup_flags & (FOLL_GET | FOLL_PIN))) because the checking
> has been done in is_valid_gup_args() for external users of __get_user_pages().

I will also note that you changed the flags check to VM_WARN_ON_ONCE.
I guess this is fine as we have the WARN_ON_ONCE version in
is_valid_gup_args().

> Signed-off-by: Baoquan He <bhe@redhat.com>

LGTM,

Reviewed-by: Oscar Salvador <osalvador@suse.de>


-- 
Oscar Salvador
SUSE Labs

On 08.04.25 11:04, Oscar Salvador wrote:
> On Mon, Apr 07, 2025 at 11:03:05AM +0800, Baoquan He wrote:
>> In __get_user_pages(), it will traverse page table and take a reference
>> to the page the given user address corresponds to if GUP_GET or GUP_PIN
>> is set. However, it's not supported both GUP_GET and GUP_PIN are set.
>> Even though this check need be done, it should be done earlier, but not
>> doing it till entering into follow_page_pte() and failed.
>>
>> Furthermore, this checking has been done in is_valid_gup_args() and all
>> external users of __get_user_pages() will call is_valid_gup_args() to
>> catch the illegal setting. We don't need to worry about internal users
>> of __get_user_pages() because the gup_flags are set by MM code correctly.
>>
>> Here remove the checking in follow_page_pte(), and add VM_WARN_ON_ONCE()
>> to catch the possible exceptional setting just in case.
>>
>> And also change the VM_BUG_ON to VM_WARN_ON_ONCE() for checking
>> (!!pages != !!(gup_flags & (FOLL_GET | FOLL_PIN))) because the checking
>> has been done in is_valid_gup_args() for external users of __get_user_pages().
> 
> I will also note that you changed the flags check to VM_WARN_ON_ONCE.
> I guess this is fine as we have the WARN_ON_ONCE version in
> is_valid_gup_args().
> 

Yes, that was my reasoning when I proposed it.

>> Signed-off-by: Baoquan He <bhe@redhat.com>
> 
> LGTM,
> 
> Reviewed-by: Oscar Salvador <osalvador@suse.de>
> 
> 

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb