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mm/init: Add 'rodata=off' boot cmdline parameter to disable read-only kernel mappings

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It may be useful to debug writes to the readonly sections of memory,
so provide a cmdline "rodata=off" to allow for this. This can be
expanded in the future to support "log" and "write" modes, but that
will need to be architecture-specific.

This also makes KDB software breakpoints more usable, as read-only
mappings can now be disabled on any kernel.

Suggested-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: David Brown <david.brown@linaro.org>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Emese Revfy <re.emese@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mathias Krause <minipli@googlemail.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: PaX Team <pageexec@freemail.hu>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: kernel-hardening@lists.openwall.com
Cc: linux-arch <linux-arch@vger.kernel.org>
Link: http://lkml.kernel.org/r/1455748879-21872-3-git-send-email-keescook@chromium.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Kees Cook 2016-02-17 14:41:13 -08:00 committed by Ingo Molnar
parent e267d97b83
commit d2aa1acad2
3 changed files with 28 additions and 7 deletions
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4
Documentation/kernel-parameters.txt
View File

@ -3491,6 +3491,10 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
ro [KNL] Mount root device read-only on boot
rodata= [KNL]
on Mark read-only kernel memory as read-only (default).
off Leave read-only kernel memory writable for debugging.
root= [KNL] Root filesystem
See name_to_dev_t comment in init/do_mounts.c.

27
init/main.c
View File

@ -93,9 +93,6 @@ static int kernel_init(void *);
extern void init_IRQ(void);
extern void fork_init(void);
extern void radix_tree_init(void);
#ifndef CONFIG_DEBUG_RODATA
static inline void mark_rodata_ro(void) { }
#endif
/*
* Debug helper: via this flag we know that we are in 'early bootup code'
@ -929,6 +926,28 @@ static int try_to_run_init_process(const char *init_filename)
static noinline void __init kernel_init_freeable(void);
#ifdef CONFIG_DEBUG_RODATA
static bool rodata_enabled = true;
static int __init set_debug_rodata(char *str)
{
return strtobool(str, &rodata_enabled);
}
__setup("rodata=", set_debug_rodata);
static void mark_readonly(void)
{
if (rodata_enabled)
mark_rodata_ro();
else
pr_info("Kernel memory protection disabled.\n");
}
#else
static inline void mark_readonly(void)
{
pr_warn("This architecture does not have kernel memory protection.\n");
}
#endif
static int __ref kernel_init(void *unused)
{
int ret;
@ -937,7 +956,7 @@ static int __ref kernel_init(void *unused)
/* need to finish all async __init code before freeing the memory */
async_synchronize_full();
free_initmem();
mark_rodata_ro();
mark_readonly();
system_state = SYSTEM_RUNNING;
numa_default_policy();

4
kernel/debug/kdb/kdb_bp.c
View File

@ -153,13 +153,11 @@ static int _kdb_bp_install(struct pt_regs *regs, kdb_bp_t *bp)
} else {
kdb_printf("%s: failed to set breakpoint at 0x%lx\n",
__func__, bp->bp_addr);
#ifdef CONFIG_DEBUG_RODATA
if (!bp->bp_type) {
kdb_printf("Software breakpoints are unavailable.\n"
" Change the kernel CONFIG_DEBUG_RODATA=n\n"
" Boot the kernel with rodata=off\n"
" OR use hw breaks: help bph\n");
}
#endif
return 1;
}
return 0;