From: Joerg Roedel <jroedel@xxxxxxx>
One can not simply remove vmalloc faulting on x86-32. Upstream
commit: 7f0a002b5a21 ("x86/mm: remove vmalloc faulting")
removed it on x86 alltogether because previously the
arch_sync_kernel_mappings() interface was introduced. This interface
added synchronization of vmalloc/ioremap page-table updates to all
page-tables in the system at creation time and was thought to make
vmalloc faulting obsolete.
But that assumption was incredibly naive.
It turned out that there is a race window between the time the vmalloc
or ioremap code establishes a mapping and the time it synchronizes
this change to other page-tables in the system.
During this race window another CPU or thread can establish a vmalloc
mapping which uses the same intermediate page-table entries (e.g. PMD
or PUD) and does no synchronization in the end, because it found all
necessary mappings already present in the kernel reference page-table.
But when these intermediate page-table entries are not yet
synchronized, the other CPU or thread will continue with a vmalloc
address that is not yet mapped in the page-table it currently uses,
causing an unhandled page fault and oops like below:
BUG: unable to handle page fault for address: fe80c000
#PF: supervisor write access in kernel mode
#PF: error_code(0x0002) - not-present page
*pde = 33183067 *pte = a8648163
Oops: 0002 [#1] SMP
CPU: 1 PID: 13514 Comm: cve-2017-17053 Tainted: G
0-next-20200811 #1
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
4/01/2014
EIP: memcpy+0xf/0x20
Code: 68 d0 7d ee d6 e8 11 1c c7 ff 0f 31 31 c3 59 58 cc cc cc cc cc cc 55 89 e5 57 89 c7 56 89 d6 53 89 cb a5 89 d9 83 e1 03 74 02 f3 a4 5b 5e 5f 5d c3 90 55 89 e5
EAX: fe80c000 EBX: 00010000 ECX: 00004000 EDX: fbfbd000
ESI: fbfbd000 EDI: fe80c000 EBP: f11f1e2c ESP: f11f1e20
DS: 007b ES: 007b FS: 00d8 GS: 00e0 SS: 0068 EFLAGS: 00010216
CR0: 80050033 CR2: fe80c000 CR3: 314c0000 CR4: 003506d0
DR0: 00000000 DR1: 00000000 DR2: 00000000 DR3: 00000000
DR6: ffff4ff0 DR7: 00000400
Call Trace:
ldt_dup_context+0x66/0x80
dup_mm+0x2b3/0x480
copy_process+0x133b/0x15c0
_do_fork+0x94/0x3e0
__ia32_sys_clone+0x67/0x80
__do_fast_syscall_32+0x3f/0x70
do_fast_syscall_32+0x29/0x60
do_SYSENTER_32+0x15/0x20
entry_SYSENTER_32+0x9f/0xf2
EIP: 0xb7eef549
So the arch_sync_kernel_mappings() interface is racy, but removing it
would mean to re-introduce the vmalloc_sync_all() interface, which is
even more awful. Keep arch_sync_kernel_mappings() in place and catch
the race condition in the page-fault handler instead.
Do a partial revert of above commit to get vmalloc faulting on x86-32
back in place.
Reported-by: Naresh Kamboju <naresh.kamboju@xxxxxxxxxx>
Fixes: 7f0a002b5a21 ("x86/mm: remove vmalloc faulting")
Cc: stable@xxxxxxxxxxxxxxx # v5.8+
Signed-off-by: Joerg Roedel <jroedel@xxxxxxx>
---
arch/x86/mm/fault.c | 78 +++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 78 insertions(+)
diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 35f1498e9832..6e3e8a124903 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -190,6 +190,53 @@ static inline pmd_t *vmalloc_sync_one(pgd_t *pgd, unsigned long address)
return pmd_k;
}
+/*
+ * Handle a fault on the vmalloc or module mapping area
+ *
+ * This is needed because there is a race condition between the time
+ * when the vmalloc mapping code updates the PMD to the point in time
+ * where it synchronizes this update with the other page-tables in the
+ * system.
+ *
+ * In this race window another thread/CPU can map an area on the same
+ * PMD, finds it already present and does not synchronize it with the
+ * rest of the system yet. As a result v[mz]alloc might return areas
+ * which are not mapped in every page-table in the system, causing an
+ * unhandled page-fault when they are accessed.
+ */
+static noinline int vmalloc_fault(unsigned long address)
+{
+ unsigned long pgd_paddr;
+ pmd_t *pmd_k;
+ pte_t *pte_k;
+
+ /* Make sure we are in vmalloc area: */
+ if (!(address >= VMALLOC_START && address < VMALLOC_END))
+ return -1;
+
+ /*
+ * Synchronize this task's top level page-table
+ * with the 'reference' page table.
+ *
+ * Do _not_ use "current" here. We might be inside
+ * an interrupt in the middle of a task switch..
+ */
+ pgd_paddr = read_cr3_pa();
+ pmd_k = vmalloc_sync_one(__va(pgd_paddr), address);
+ if (!pmd_k)
+ return -1;
+
+ if (pmd_large(*pmd_k))
+ return 0;
+
+ pte_k = pte_offset_kernel(pmd_k, address);
+ if (!pte_present(*pte_k))
+ return -1;
+
+ return 0;
+}
+NOKPROBE_SYMBOL(vmalloc_fault);
+
void arch_sync_kernel_mappings(unsigned long start, unsigned long end)
{
unsigned long addr;
@@ -1110,6 +1157,37 @@ do_kern_addr_fault(struct pt_regs *regs, unsigned long hw_error_code,
*/
WARN_ON_ONCE(hw_error_code & X86_PF_PK);
+#ifdef CONFIG_X86_32
+ /*
+ * We can fault-in kernel-space virtual memory on-demand. The
+ * 'reference' page table is init_mm.pgd.
+ *
+ * NOTE! We MUST NOT take any locks for this case. We may
+ * be in an interrupt or a critical region, and should
+ * only copy the information from the master page table,
+ * nothing more.
+ *
+ * Before doing this on-demand faulting, ensure that the
+ * fault is not any of the following:
+ * 1. A fault on a PTE with a reserved bit set.
+ * 2. A fault caused by a user-mode access. (Do not demand-
+ * fault kernel memory due to user-mode accesses).
+ * 3. A fault caused by a page-level protection violation.
+ * (A demand fault would be on a non-present page which
+ * would have X86_PF_PROT==0).
+ *
+ * This is only needed to close a race condition on x86-32 in
+ * the vmalloc mapping/unmapping code. See the comment above
+ * vmalloc_fault() for details. On x86-64 the race does not
+ * exist as the vmalloc mappings don't need to be synchronized
+ * there.
+ */
+ if (!(hw_error_code & (X86_PF_RSVD | X86_PF_USER | X86_PF_PROT))) {
+ if (vmalloc_fault(address) >= 0)
+ return;
+ }
+#endif
+
/* Was the fault spurious, caused by lazy TLB invalidation? */
if (spurious_kernel_fault(hw_error_code, address))
return;
--
2.28.0
