From: Nadav Amit <namit@xxxxxxxxxx>
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 <jthoughton@xxxxxxxxxx>
Cc: Peter Xu <peterx@xxxxxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Mike Rapoport <rppt@xxxxxxxxxxxxx>
Cc: Jan Kara <jack@xxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Nadav Amit <namit@xxxxxxxxxx>
---
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
