linux-kernel - [PATCH] riscv: mm: Proper page permissions after initmem free

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Message-Id: <20221112113543.3165646-1-bjorn@kernel.org>
Date:   Sat, 12 Nov 2022 12:35:43 +0100
From:   Björn Töpel <bjorn@...nel.org>
To:     Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        linux-riscv@...ts.infradead.org, Alexandre Ghiti <alex@...ti.fr>
Cc:     Björn Töpel <bjorn@...osinc.com>,
        linux-kernel@...r.kernel.org
Subject: [PATCH] riscv: mm: Proper page permissions after initmem free

From: Björn Töpel <bjorn@...osinc.com>

64-bit RISC-V kernels have the kernel image mapped separately, and in
addition to the linear map. When the kernel is loaded, the linear map
of kernel image is set to PAGE_READ permission, and the kernel map is
set to PAGE_READ and PAGE_EXEC.

When the initmem is freed, the corresponding pages in the linear map
should be restored to PAGE_READ and PAGE_WRITE. The corresponding
pages in the kernel map should also be restored to PAGE_READ and
PAGE_WRITE, by removing the PAGE_EXEC permission, and adding
PAGE_WRITE.

This is not the case. For 64-bit kernels, only the linear map is
restored to its proper page permissions at initmem free, and not the
kernelmap.

In practise this results in that the kernel can potentially jump to
dead __init code, and start executing invalid 0xcc instructions,
without getting an exception.

Restore the freed initmem properly, by setting both the alias (kernel
map) and the linear map to the correct permissions.

Fixes: e5c35fa04019 ("riscv: Map the kernel with correct permissions the first time")
Signed-off-by: Björn Töpel <bjorn@...osinc.com>
---
 arch/riscv/kernel/setup.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/arch/riscv/kernel/setup.c b/arch/riscv/kernel/setup.c
index ad76bb59b059..361e635070fe 100644
--- a/arch/riscv/kernel/setup.c
+++ b/arch/riscv/kernel/setup.c
@@ -321,10 +321,12 @@ subsys_initcall(topology_init);

 void free_initmem(void)
 {
-	if (IS_ENABLED(CONFIG_STRICT_KERNEL_RWX))
-		set_kernel_memory(lm_alias(__init_begin), lm_alias(__init_end),
-				  IS_ENABLED(CONFIG_64BIT) ?
-					set_memory_rw : set_memory_rw_nx);
+	if (IS_ENABLED(CONFIG_STRICT_KERNEL_RWX)) {
+		if (IS_ENABLED(CONFIG_64BIT))
+			set_kernel_memory(lm_alias(__init_begin), lm_alias(__init_end),
+					  set_memory_rw);
+		set_kernel_memory(__init_begin, __init_end, set_memory_rw_nx);
+	}

 	free_initmem_default(POISON_FREE_INITMEM);
 }

base-commit: 442bcbfd2c5401587b983e34bed0b407214735c3
-- 
2.37.2