Re: [PATCH] mm: page_alloc: fix page_poison=1 / INIT_ON_ALLOC_DEFAULT_ON interaction

[Kees Cook <keescook@chromium.org>]

On Mon, Jul 12, 2021 at 10:58:16PM +0100, Sergei Trofimovich wrote:
> To reproduce the failure we need the following system:
>   - kernel command: page_poison=1 init_on_free=0 init_on_alloc=0
>   - kernel config:
>     * CONFIG_INIT_ON_ALLOC_DEFAULT_ON=y
>     * CONFIG_INIT_ON_FREE_DEFAULT_ON=y
>     * CONFIG_PAGE_POISONING=y
> 
>     0000000085629bdd: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
>     0000000022861832: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
>     00000000c597f5b0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
>     CPU: 11 PID: 15195 Comm: bash Kdump: loaded Tainted: G     U     O      5.13.1-gentoo-x86_64 #1
>     Hardware name: System manufacturer System Product Name/PRIME Z370-A, BIOS 2801 01/13/2021
>     Call Trace:
>      dump_stack+0x64/0x7c
>      __kernel_unpoison_pages.cold+0x48/0x84
>      post_alloc_hook+0x60/0xa0
>      get_page_from_freelist+0xdb8/0x1000
>      __alloc_pages+0x163/0x2b0
>      __get_free_pages+0xc/0x30
>      pgd_alloc+0x2e/0x1a0
>      ? dup_mm+0x37/0x4f0
>      mm_init+0x185/0x270
>      dup_mm+0x6b/0x4f0
>      ? __lock_task_sighand+0x35/0x70
>      copy_process+0x190d/0x1b10
>      kernel_clone+0xba/0x3b0
>      __do_sys_clone+0x8f/0xb0
>      do_syscall_64+0x68/0x80
>      ? do_syscall_64+0x11/0x80
>      entry_SYSCALL_64_after_hwframe+0x44/0xae
> 
> Before the 51cba1eb ("init_on_alloc: Optimize static branches")
> init_on_alloc never enabled static branch by default. It could
> only be enabed explicitly by init_mem_debugging_and_hardening().

But init_mem_debugging_and_hardening() is always called (by mm_init()).

> But after the 51cba1eb static branch could already be enabled
> by default. There was no code to ever disable it. That caused
> page_poison=1 / init_on_free=1 conflict.
> 
> This change extends init_mem_debugging_and_hardening() to also
> disable static branch disabling.
> 
> CC: Andrew Morton <akpm@linux-foundation.org>
> CC: Kees Cook <keescook@chromium.org>
> CC: Alexander Potapenko <glider@google.com>
> CC: Thomas Gleixner <tglx@linutronix.de>
> CC: Vlastimil Babka <vbabka@suse.cz>
> CC: linux-mm@kvack.org
> Reported-by: bowsingbetee@pm.me
> Reported-by: Mikhail Morfikov
> Fixes: 51cba1eb ("init_on_alloc: Optimize static branches")
> Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>
> ---
>  mm/page_alloc.c | 16 ++++++++++------
>  1 file changed, 10 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 3b97e17806be..46cb4a9c2b50 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -840,18 +840,22 @@ void init_mem_debugging_and_hardening(void)
>  	}
>  #endif
>  
> -	if (_init_on_alloc_enabled_early) {
> -		if (page_poisoning_requested)
> +	if (_init_on_alloc_enabled_early ||
> +	    IS_ENABLED(CONFIG_INIT_ON_ALLOC_DEFAULT_ON)) {

This doesn't look right. _init_on_alloc_enabled_early already has the
same value:

static bool _init_on_alloc_enabled_early __read_mostly
                                = IS_ENABLED(CONFIG_INIT_ON_ALLOC_DEFAULT_ON);

So checking this is just a side-effect of how static_branch_maybe()
happens to be behaving.

> +		if (page_poisoning_requested) {
>  			pr_info("mem auto-init: CONFIG_PAGE_POISONING is on, "
>  				"will take precedence over init_on_alloc\n");
> -		else
> +			static_branch_disable(&init_on_alloc);
> +		} else
>  			static_branch_enable(&init_on_alloc);
>  	}
> -	if (_init_on_free_enabled_early) {
> -		if (page_poisoning_requested)
> +	if (_init_on_free_enabled_early ||
> +	    IS_ENABLED(CONFIG_INIT_ON_FREE_DEFAULT_ON)) {
> +		if (page_poisoning_requested) {
>  			pr_info("mem auto-init: CONFIG_PAGE_POISONING is on, "
>  				"will take precedence over init_on_free\n");
> -		else
> +			static_branch_disable(&init_on_free);
> +		} else
>  			static_branch_enable(&init_on_free);
>  	}

I think it would be better to clean this up without additional
confusion involving the CONFIGs:

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 3b97e17806be..1f19365bc158 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -840,21 +840,24 @@ void init_mem_debugging_and_hardening(void)
 	}
 #endif
 
-	if (_init_on_alloc_enabled_early) {
-		if (page_poisoning_requested)
-			pr_info("mem auto-init: CONFIG_PAGE_POISONING is on, "
-				"will take precedence over init_on_alloc\n");
-		else
-			static_branch_enable(&init_on_alloc);
-	}
-	if (_init_on_free_enabled_early) {
-		if (page_poisoning_requested)
-			pr_info("mem auto-init: CONFIG_PAGE_POISONING is on, "
-				"will take precedence over init_on_free\n");
-		else
-			static_branch_enable(&init_on_free);
+	if ((_init_on_alloc_enabled_early || _init_on_free_enabled_early) &&
+	    page_poisoning_requested) {
+		pr_info("mem auto-init: CONFIG_PAGE_POISONING is on, "
+			"will take precedence over init_on_alloc and init_on_free\n");
+		_init_on_alloc_enabled_early = false;
+		_init_on_free_enabled_early = false;
 	}
 
+	if (_init_on_alloc_enabled_early)
+		static_branch_enable(&init_on_alloc);
+	else
+		static_branch_disable(&init_on_alloc);
+
+	if (_init_on_free_enabled_early)
+		static_branch_enable(&init_on_free);
+	else
+		static_branch_disable(&init_on_free);
+
 #ifdef CONFIG_DEBUG_PAGEALLOC
 	if (!debug_pagealloc_enabled())
 		return;

-- 
Kees Cook