From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 49E6F40DFA3 for ; Sun, 3 May 2026 11:42:53 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777808573; cv=none; b=C82CFIVbyy1HpIKpHRV/fgWWj5bn2XTHaXpsDw+zCVCrjKQg/K/R+iLdCfdtzCMIb2KegHV27bb0mgRtjV21jnYghmdGpjfa6PT69T+HRqrYDIvMaxCtk/te/u9TBPPXJyBbgHkZ5Ei1UruiDsMo8j1Padu/nzFPHx26P6DnATg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777808573; c=relaxed/simple; bh=H9mvTyDfP++tKmW/mJsytfc/kxaRhnUolB0JjISzy9g=; h=Subject:To:Cc:From:Date:Message-ID:MIME-Version:Content-Type; b=fd/Njh88bT8juls67VvpYGCE/JK1Bmj3kzVsah+v+NJ7a2DC9KeFq1lZ56W6aEwP0qaUy+HbeUsC5i0FwPrqzFOpRBN/GM5Ju7BGMe7mH677KdSNl9WSEbk40AtsHXRwSS96ADV9YifW1EGFl/JKKYIUJW/SqpSVqytzJOQqVvs= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=1BLAH/hS; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="1BLAH/hS" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 78DA1C2BCB4; Sun, 3 May 2026 11:42:52 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1777808572; bh=H9mvTyDfP++tKmW/mJsytfc/kxaRhnUolB0JjISzy9g=; h=Subject:To:Cc:From:Date:From; b=1BLAH/hSW4t+IxL30EvupP9zX0D50YVoT7EnCLr3yvDjMK9U5l3uakqmhwu8WsAMx uz5DJzzTrXfXIn1ZzOrzd4uasimlhDKaQOmHzabEREmb5uPlCuz8uNjcXBFQ/qDOm7 RRirBc+afZepwVH56Iq2fxwOi4x2kFr1YSIpjcwg= Subject: FAILED: patch "[PATCH] randomize_kstack: Maintain kstack_offset per task" failed to apply to 5.15-stable tree To: ryan.roberts@arm.com,kees@kernel.org,mark.rutland@arm.com Cc: From: Date: Sun, 03 May 2026 13:42:50 +0200 Message-ID: <2026050350-sensation-oink-0da0@gregkh> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=ANSI_X3.4-1968 Content-Transfer-Encoding: 8bit The patch below does not apply to the 5.15-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to . To reproduce the conflict and resubmit, you may use the following commands: git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y git checkout FETCH_HEAD git cherry-pick -x 37beb42560165869838e7d91724f3e629db64129 # git commit -s git send-email --to '' --in-reply-to '2026050350-sensation-oink-0da0@gregkh' --subject-prefix 'PATCH 5.15.y' HEAD^.. Possible dependencies: thanks, greg k-h ------------------ original commit in Linus's tree ------------------ >From 37beb42560165869838e7d91724f3e629db64129 Mon Sep 17 00:00:00 2001 From: Ryan Roberts Date: Tue, 3 Mar 2026 15:08:38 +0000 Subject: [PATCH] randomize_kstack: Maintain kstack_offset per task kstack_offset was previously maintained per-cpu, but this caused a couple of issues. So let's instead make it per-task. Issue 1: add_random_kstack_offset() and choose_random_kstack_offset() expected and required to be called with interrupts and preemption disabled so that it could manipulate per-cpu state. But arm64, loongarch and risc-v are calling them with interrupts and preemption enabled. I don't _think_ this causes any functional issues, but it's certainly unexpected and could lead to manipulating the wrong cpu's state, which could cause a minor performance degradation due to bouncing the cache lines. By maintaining the state per-task those functions can safely be called in preemptible context. Issue 2: add_random_kstack_offset() is called before executing the syscall and expands the stack using a previously chosen random offset. choose_random_kstack_offset() is called after executing the syscall and chooses and stores a new random offset for the next syscall. With per-cpu storage for this offset, an attacker could force cpu migration during the execution of the syscall and prevent the offset from being updated for the original cpu such that it is predictable for the next syscall on that cpu. By maintaining the state per-task, this problem goes away because the per-task random offset is updated after the syscall regardless of which cpu it is executing on. Fixes: 39218ff4c625 ("stack: Optionally randomize kernel stack offset each syscall") Closes: https://lore.kernel.org/all/dd8c37bc-795f-4c7a-9086-69e584d8ab24@arm.com/ Cc: stable@vger.kernel.org Acked-by: Mark Rutland Signed-off-by: Ryan Roberts Link: https://patch.msgid.link/20260303150840.3789438-2-ryan.roberts@arm.com Signed-off-by: Kees Cook diff --git a/include/linux/randomize_kstack.h b/include/linux/randomize_kstack.h index 1d982dbdd0d0..5d3916ca747c 100644 --- a/include/linux/randomize_kstack.h +++ b/include/linux/randomize_kstack.h @@ -9,7 +9,6 @@ DECLARE_STATIC_KEY_MAYBE(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, randomize_kstack_offset); -DECLARE_PER_CPU(u32, kstack_offset); /* * Do not use this anywhere else in the kernel. This is used here because @@ -50,15 +49,14 @@ DECLARE_PER_CPU(u32, kstack_offset); * add_random_kstack_offset - Increase stack utilization by previously * chosen random offset * - * This should be used in the syscall entry path when interrupts and - * preempt are disabled, and after user registers have been stored to - * the stack. For testing the resulting entropy, please see: - * tools/testing/selftests/lkdtm/stack-entropy.sh + * This should be used in the syscall entry path after user registers have been + * stored to the stack. Preemption may be enabled. For testing the resulting + * entropy, please see: tools/testing/selftests/lkdtm/stack-entropy.sh */ #define add_random_kstack_offset() do { \ if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \ &randomize_kstack_offset)) { \ - u32 offset = raw_cpu_read(kstack_offset); \ + u32 offset = current->kstack_offset; \ u8 *ptr = __kstack_alloca(KSTACK_OFFSET_MAX(offset)); \ /* Keep allocation even after "ptr" loses scope. */ \ asm volatile("" :: "r"(ptr) : "memory"); \ @@ -69,9 +67,9 @@ DECLARE_PER_CPU(u32, kstack_offset); * choose_random_kstack_offset - Choose the random offset for the next * add_random_kstack_offset() * - * This should only be used during syscall exit when interrupts and - * preempt are disabled. This position in the syscall flow is done to - * frustrate attacks from userspace attempting to learn the next offset: + * This should only be used during syscall exit. Preemption may be enabled. This + * position in the syscall flow is done to frustrate attacks from userspace + * attempting to learn the next offset: * - Maximize the timing uncertainty visible from userspace: if the * offset is chosen at syscall entry, userspace has much more control * over the timing between choosing offsets. "How long will we be in @@ -85,14 +83,20 @@ DECLARE_PER_CPU(u32, kstack_offset); #define choose_random_kstack_offset(rand) do { \ if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \ &randomize_kstack_offset)) { \ - u32 offset = raw_cpu_read(kstack_offset); \ + u32 offset = current->kstack_offset; \ offset = ror32(offset, 5) ^ (rand); \ - raw_cpu_write(kstack_offset, offset); \ + current->kstack_offset = offset; \ } \ } while (0) + +static inline void random_kstack_task_init(struct task_struct *tsk) +{ + tsk->kstack_offset = 0; +} #else /* CONFIG_RANDOMIZE_KSTACK_OFFSET */ #define add_random_kstack_offset() do { } while (0) #define choose_random_kstack_offset(rand) do { } while (0) +#define random_kstack_task_init(tsk) do { } while (0) #endif /* CONFIG_RANDOMIZE_KSTACK_OFFSET */ #endif diff --git a/include/linux/sched.h b/include/linux/sched.h index a7b4a980eb2f..8358e430dd7f 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -1592,6 +1592,10 @@ struct task_struct { unsigned long prev_lowest_stack; #endif +#ifdef CONFIG_RANDOMIZE_KSTACK_OFFSET + u32 kstack_offset; +#endif + #ifdef CONFIG_X86_MCE void __user *mce_vaddr; __u64 mce_kflags; diff --git a/init/main.c b/init/main.c index 1cb395dd94e4..0a1d8529212e 100644 --- a/init/main.c +++ b/init/main.c @@ -833,7 +833,6 @@ static inline void initcall_debug_enable(void) #ifdef CONFIG_RANDOMIZE_KSTACK_OFFSET DEFINE_STATIC_KEY_MAYBE_RO(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, randomize_kstack_offset); -DEFINE_PER_CPU(u32, kstack_offset); static int __init early_randomize_kstack_offset(char *buf) { diff --git a/kernel/fork.c b/kernel/fork.c index 65113a304518..5715adeb6adf 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -95,6 +95,7 @@ #include #include #include +#include #include #include #include @@ -2233,6 +2234,7 @@ __latent_entropy struct task_struct *copy_process( if (retval) goto bad_fork_cleanup_io; + random_kstack_task_init(p); stackleak_task_init(p); if (pid != &init_struct_pid) {