From: Joerg Roedel jroedel@suse.de

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@linaro.org Fixes: 7f0a002b5a21 ("x86/mm: remove vmalloc faulting") Cc: stable@vger.kernel.org # v5.8+ Signed-off-by: Joerg Roedel jroedel@suse.de --- 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;