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

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Message-Id: <20231215121431.680-3-petrtesarik@huaweicloud.com>
Date: Fri, 15 Dec 2023 13:14:31 +0100
From: Petr Tesarik <petrtesarik@...weicloud.com>
To: Richard Weinberger <richard@....at>,
	Anton Ivanov <anton.ivanov@...bridgegreys.com>,
	Johannes Berg <johannes@...solutions.net>,
	linux-um@...ts.infradead.org (open list:USER-MODE LINUX (UML)),
	linux-kernel@...r.kernel.org (open list)
Cc: Roberto Sassu <roberto.sassu@...weicloud.com>,
	petr@...arici.cz,
	Petr Tesarik <petr.tesarik1@...wei-partners.com>
Subject: [PATCH 2/2] um: oops on accessing an non-present page in the vmalloc area

From: Petr Tesarik <petr.tesarik1@...wei-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.tesarik1@...wei-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