[PATCH 2/2] um: oops on accessing an non-present page in the vmalloc area

[Petr Tesarik]

From: Petr Tesarik <petr.tesar...@huawei-partners.com>

If a segmentation fault is caused by an address in the vmalloc area, check
that the target page is present.

Currently, if the kernel hits a guard page the vmalloc area, UML assumes
that the fault is caused merely by a stale mapping and will be fixed by
flush_tlb_kernel_vm(). Of course, this will not create any mapping for a
guard page, so the faulting instruction will cause exactly the same fault
when it is executed again, effectively creating a beautiful (but useless)
infinite loop.

Signed-off-by: Petr Tesarik <petr.tesar...@huawei-partners.com>
---
 arch/um/kernel/trap.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/um/kernel/trap.c b/arch/um/kernel/trap.c
index 1124a382fd14..ca9b5fd83c52 100644
--- a/arch/um/kernel/trap.c
+++ b/arch/um/kernel/trap.c
@@ -214,11 +214,15 @@ unsigned long segv(struct faultinfo fi, unsigned long ip, 
int is_user,
        int err;
        int is_write = FAULT_WRITE(fi);
        unsigned long address = FAULT_ADDRESS(fi);
+       pte_t *pte;
 
        if (!is_user && regs)
                current->thread.segv_regs = container_of(regs, struct pt_regs, 
regs);
 
        if (!is_user && (address >= start_vm) && (address < end_vm)) {
+               pte = virt_to_pte(&init_mm, address);
+               if (!pte_present(*pte))
+                       page_fault_oops(regs, address, ip);
                flush_tlb_kernel_vm();
                goto out;
        }
-- 
2.34.1