From db27ad8b0209e892a9b6294aadcf9d756f41a656 Mon Sep 17 00:00:00 2001 From: Petr Tesarik Date: Tue, 19 Nov 2024 12:37:38 +0100 Subject: [PATCH] mm/rodata_test: use READ_ONCE() to read const variable Patch series "Fix mm/rodata_test", v2. Make sure that the test actually reads the read-only memory location. Verify that the variable contains the expected value rather than any non-zero value. This patch (of 2): The C compiler may optimize away the memory read of a const variable if its value is known at compile time. In particular, GCC14 with -O2 generates no code at all for test 1, and it generates the following x86_64 instructions for test 3: cmpl $195, 4(%rsp) je .L14 That is, it replaces the read of rodata_test_data with an immediate value and compares it to the value of the local variable "zero". Use READ_ONCE() to undo any such compiler optimizations and enforce a memory read. Link: https://lkml.kernel.org/r/cover.1732016064.git.ptesarik@suse.com Link: https://lkml.kernel.org/r/2a66dee010151b25cb143efb39091ef7530aa00a.1732016064.git.ptesarik@suse.com Fixes: 2959a5f726f6 ("mm: add arch-independent testcases for RODATA") Signed-off-by: Petr Tesarik Reviewed-by: Kees Cook Cc: Jinbum Park Signed-off-by: Andrew Morton --- mm/rodata_test.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/mm/rodata_test.c b/mm/rodata_test.c index 6d783436951f..3b60425d80fe 100644 --- a/mm/rodata_test.c +++ b/mm/rodata_test.c @@ -20,7 +20,7 @@ void rodata_test(void) /* test 1: read the value */ /* If this test fails, some previous testrun has clobbered the state */ - if (!rodata_test_data) { + if (!READ_ONCE(rodata_test_data)) { pr_err("test 1 fails (start data)\n"); return; } @@ -33,7 +33,7 @@ void rodata_test(void) } /* test 3: check the value hasn't changed */ - if (rodata_test_data == zero) { + if (READ_ONCE(rodata_test_data) == zero) { pr_err("test data was changed\n"); return; } -- 2.25.1