From: Sean Christopherson seanjc@google.com Subject: mm/secretmem: fix NULL page->mapping dereference in page_is_secretmem()

Check for a NULL page->mapping before dereferencing the mapping in page_is_secretmem(), as the page's mapping can be nullified while gup() is running, e.g. by reclaim or truncation.

BUG: kernel NULL pointer dereference, address: 0000000000000068 #PF: supervisor read access in kernel mode #PF: error_code(0x0000) - not-present page PGD 0 P4D 0 Oops: 0000 [#1] PREEMPT SMP NOPTI CPU: 6 PID: 4173897 Comm: CPU 3/KVM Tainted: G W RIP: 0010:internal_get_user_pages_fast+0x621/0x9d0 Code: <48> 81 7a 68 80 08 04 bc 0f 85 21 ff ff 8 89 c7 be RSP: 0018:ffffaa90087679b0 EFLAGS: 00010046 RAX: ffffe3f37905b900 RBX: 00007f2dd561e000 RCX: ffffe3f37905b934 RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffe3f37905b900 ... CR2: 0000000000000068 CR3: 00000004c5898003 CR4: 00000000001726e0 Call Trace: get_user_pages_fast_only+0x13/0x20 hva_to_pfn+0xa9/0x3e0 try_async_pf+0xa1/0x270 direct_page_fault+0x113/0xad0 kvm_mmu_page_fault+0x69/0x680 vmx_handle_exit+0xe1/0x5d0 kvm_arch_vcpu_ioctl_run+0xd81/0x1c70 kvm_vcpu_ioctl+0x267/0x670 __x64_sys_ioctl+0x83/0xa0 do_syscall_64+0x56/0x80 entry_SYSCALL_64_after_hwframe+0x44/0xae

Link: https://lkml.kernel.org/r/20211007231502.3552715-1-seanjc@google.com Fixes: 1507f51255c9 ("mm: introduce memfd_secret system call to create "secret" memory areas") Signed-off-by: Sean Christopherson seanjc@google.com Reported-by: Darrick J. Wong djwong@kernel.org Reported-by: Stephen stephenackerman16@gmail.com Tested-by: Darrick J. Wong djwong@kernel.org Reviewed-by: David Hildenbrand david@redhat.com Reviewed-by: Mike Rapoport rppt@linux.ibm.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

include/linux/secretmem.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/include/linux/secretmem.h~mm-fix-null-page-mapping-dereference-in-page_is_secretmem +++ a/include/linux/secretmem.h @@ -23,7 +23,7 @@ static inline bool page_is_secretmem(str mapping = (struct address_space *) ((unsigned long)page->mapping & ~PAGE_MAPPING_FLAGS);

- if (mapping != page->mapping) + if (!mapping || mapping != page->mapping) return false;

return mapping->a_ops == &secretmem_aops; _