userfaultfd: provide properly masked address for huge-pages

Commit Message

Nadav Amit July 11, 2022, 4:59 p.m. UTC

From: Nadav Amit <[e-mail]>

Commit 824ddc601adc ("userfaultfd: provide unmasked address on
page-fault") was introduced to fix an old bug, in which the offset in
the address of a page-fault was masked. Concerns were raised - although
were never backed by actual code - that some userspace code might break
because the bug has been around for quite a while. To address these
concerns a new flag was introduced, and only when this flag is set by
the user, userfaultfd provides the exact address of the page-fault.

The commit however had a bug, and if the flag is unset, the offset was
always masked based on a base-page granularity. Yet, for huge-pages, the
behavior prior to the commit was that the address is masked to the
huge-page granulrity.

While there are no reports on real breakage, fix this issue. If the flag
is unset, use the address with the masking that was done before.

Fixes: 824ddc601adc ("userfaultfd: provide unmasked address on page-fault")
Reported-by: James Houghton <[e-mail]>
Cc: Peter Xu <[e-mail]>
Cc: David Hildenbrand <[e-mail]>
Cc: Mike Rapoport <[e-mail]>
Cc: Jan Kara <[e-mail]>
Cc: Andrea Arcangeli <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Nadav Amit <[e-mail]>
---
 fs/userfaultfd.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

Comments

Mike Rapoport July 12, 2022, 6:33 a.m. UTC | #1

On Mon, Jul 11, 2022 at 09:59:06AM -0700, Nadav Amit wrote:
> From: Nadav Amit <[e-mail]>
> 
> Commit 824ddc601adc ("userfaultfd: provide unmasked address on
> page-fault") was introduced to fix an old bug, in which the offset in
> the address of a page-fault was masked. Concerns were raised - although
> were never backed by actual code - that some userspace code might break
> because the bug has been around for quite a while. To address these
> concerns a new flag was introduced, and only when this flag is set by
> the user, userfaultfd provides the exact address of the page-fault.
> 
> The commit however had a bug, and if the flag is unset, the offset was
> always masked based on a base-page granularity. Yet, for huge-pages, the
> behavior prior to the commit was that the address is masked to the
> huge-page granulrity.
> 
> While there are no reports on real breakage, fix this issue. If the flag
> is unset, use the address with the masking that was done before.
> 
> Fixes: 824ddc601adc ("userfaultfd: provide unmasked address on page-fault")
> Reported-by: James Houghton <[e-mail]>
> Cc: Peter Xu <[e-mail]>
> Cc: David Hildenbrand <[e-mail]>
> Cc: Mike Rapoport <[e-mail]>
> Cc: Jan Kara <[e-mail]>
> Cc: Andrea Arcangeli <[e-mail]>
> Cc: <[e-mail]>
> Signed-off-by: Nadav Amit <[e-mail]>

Reviewed-by: Mike Rapoport <[e-mail]>

> ---
>  fs/userfaultfd.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index e943370107d0..de86f5b2859f 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -192,17 +192,19 @@ static inline void msg_init(struct uffd_msg *msg)
>  }
>  
>  static inline struct uffd_msg userfault_msg(unsigned long address,
> +					    unsigned long real_address,
>  					    unsigned int flags,
>  					    unsigned long reason,
>  					    unsigned int features)
>  {
>  	struct uffd_msg msg;
> +
>  	msg_init(&msg);
>  	msg.event = UFFD_EVENT_PAGEFAULT;
>  
> -	if (!(features & UFFD_FEATURE_EXACT_ADDRESS))
> -		address &= PAGE_MASK;
> -	msg.arg.pagefault.address = address;
> +	msg.arg.pagefault.address = (features & UFFD_FEATURE_EXACT_ADDRESS) ?
> +				    real_address : address;
> +
>  	/*
>  	 * These flags indicate why the userfault occurred:
>  	 * - UFFD_PAGEFAULT_FLAG_WP indicates a write protect fault.
> @@ -488,8 +490,8 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
>  
>  	init_waitqueue_func_entry(&uwq.wq, userfaultfd_wake_function);
>  	uwq.wq.private = current;
> -	uwq.msg = userfault_msg(vmf->real_address, vmf->flags, reason,
> -			ctx->features);
> +	uwq.msg = userfault_msg(vmf->address, vmf->real_address, vmf->flags,
> +				reason, ctx->features);
>  	uwq.ctx = ctx;
>  	uwq.waken = false;
>  
> -- 
> 2.25.1
>

Peter Xu July 12, 2022, 1:27 p.m. UTC | #2

On Mon, Jul 11, 2022 at 09:59:06AM -0700, Nadav Amit wrote:
> From: Nadav Amit <[e-mail]>
> 
> Commit 824ddc601adc ("userfaultfd: provide unmasked address on
> page-fault") was introduced to fix an old bug, in which the offset in
> the address of a page-fault was masked. Concerns were raised - although
> were never backed by actual code - that some userspace code might break
> because the bug has been around for quite a while. To address these
> concerns a new flag was introduced, and only when this flag is set by
> the user, userfaultfd provides the exact address of the page-fault.
> 
> The commit however had a bug, and if the flag is unset, the offset was
> always masked based on a base-page granularity. Yet, for huge-pages, the
> behavior prior to the commit was that the address is masked to the
> huge-page granulrity.
> 
> While there are no reports on real breakage, fix this issue. If the flag
> is unset, use the address with the masking that was done before.
> 
> Fixes: 824ddc601adc ("userfaultfd: provide unmasked address on page-fault")
> Reported-by: James Houghton <[e-mail]>
> Cc: Peter Xu <[e-mail]>
> Cc: David Hildenbrand <[e-mail]>
> Cc: Mike Rapoport <[e-mail]>
> Cc: Jan Kara <[e-mail]>
> Cc: Andrea Arcangeli <[e-mail]>
> Cc: <[e-mail]>
> Signed-off-by: Nadav Amit <[e-mail]>

Reviewed-by: Peter Xu <[e-mail]>

James Houghton July 12, 2022, 5:04 p.m. UTC | #3

On Mon, Jul 11, 2022 at 5:33 PM Nadav Amit <[e-mail]> wrote:
>
> From: Nadav Amit <[e-mail]>
>
> Commit 824ddc601adc ("userfaultfd: provide unmasked address on
> page-fault") was introduced to fix an old bug, in which the offset in
> the address of a page-fault was masked. Concerns were raised - although
> were never backed by actual code - that some userspace code might break
> because the bug has been around for quite a while. To address these
> concerns a new flag was introduced, and only when this flag is set by
> the user, userfaultfd provides the exact address of the page-fault.
>
> The commit however had a bug, and if the flag is unset, the offset was
> always masked based on a base-page granularity. Yet, for huge-pages, the
> behavior prior to the commit was that the address is masked to the
> huge-page granulrity.
>
> While there are no reports on real breakage, fix this issue. If the flag
> is unset, use the address with the masking that was done before.
>
> Fixes: 824ddc601adc ("userfaultfd: provide unmasked address on page-fault")
> Reported-by: James Houghton <[e-mail]>
> Cc: Peter Xu <[e-mail]>
> Cc: David Hildenbrand <[e-mail]>
> Cc: Mike Rapoport <[e-mail]>
> Cc: Jan Kara <[e-mail]>
> Cc: Andrea Arcangeli <[e-mail]>
> Cc: <[e-mail]>
> Signed-off-by: Nadav Amit <[e-mail]>

Reviewed-by: James Houghton <[e-mail]>

Thanks!

> ---
>  fs/userfaultfd.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
>
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index e943370107d0..de86f5b2859f 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -192,17 +192,19 @@ static inline void msg_init(struct uffd_msg *msg)
>  }
>
>  static inline struct uffd_msg userfault_msg(unsigned long address,
> +                                           unsigned long real_address,
>                                             unsigned int flags,
>                                             unsigned long reason,
>                                             unsigned int features)
>  {
>         struct uffd_msg msg;
> +
>         msg_init(&msg);
>         msg.event = UFFD_EVENT_PAGEFAULT;
>
> -       if (!(features & UFFD_FEATURE_EXACT_ADDRESS))
> -               address &= PAGE_MASK;
> -       msg.arg.pagefault.address = address;
> +       msg.arg.pagefault.address = (features & UFFD_FEATURE_EXACT_ADDRESS) ?
> +                                   real_address : address;
> +
>         /*
>          * These flags indicate why the userfault occurred:
>          * - UFFD_PAGEFAULT_FLAG_WP indicates a write protect fault.
> @@ -488,8 +490,8 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
>
>         init_waitqueue_func_entry(&uwq.wq, userfaultfd_wake_function);
>         uwq.wq.private = current;
> -       uwq.msg = userfault_msg(vmf->real_address, vmf->flags, reason,
> -                       ctx->features);
> +       uwq.msg = userfault_msg(vmf->address, vmf->real_address, vmf->flags,
> +                               reason, ctx->features);
>         uwq.ctx = ctx;
>         uwq.waken = false;
>
> --
> 2.25.1
>

Patch

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index e943370107d0..de86f5b2859f 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -192,17 +192,19 @@  static inline void msg_init(struct uffd_msg *msg)
 }
 
 static inline struct uffd_msg userfault_msg(unsigned long address,
+					    unsigned long real_address,
 					    unsigned int flags,
 					    unsigned long reason,
 					    unsigned int features)
 {
 	struct uffd_msg msg;
+
 	msg_init(&msg);
 	msg.event = UFFD_EVENT_PAGEFAULT;
 
-	if (!(features & UFFD_FEATURE_EXACT_ADDRESS))
-		address &= PAGE_MASK;
-	msg.arg.pagefault.address = address;
+	msg.arg.pagefault.address = (features & UFFD_FEATURE_EXACT_ADDRESS) ?
+				    real_address : address;
+
 	/*
 	 * These flags indicate why the userfault occurred:
 	 * - UFFD_PAGEFAULT_FLAG_WP indicates a write protect fault.
@@ -488,8 +490,8 @@  vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
 
 	init_waitqueue_func_entry(&uwq.wq, userfaultfd_wake_function);
 	uwq.wq.private = current;
-	uwq.msg = userfault_msg(vmf->real_address, vmf->flags, reason,
-			ctx->features);
+	uwq.msg = userfault_msg(vmf->address, vmf->real_address, vmf->flags,
+				reason, ctx->features);
 	uwq.ctx = ctx;
 	uwq.waken = false;