From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 E449238B154; Thu, 6 Aug 2026 17:32:29 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786037551; cv=none; b=KdbPhSxnoddsf34IvlBYZBGAbV7JCPji57moZevTcsY1rxAhkI8VsVUAlj2borZjiwiymTseO/bRIt68dL11Wt/7cw+klHO70vfOzkPPtC0KihWKX73C5VUhTHGA/knuDllZi4ayCfPSh30w6R9BwJavRnYaDETspurP3QBjDo0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786037551; c=relaxed/simple; bh=h0nIUHYaLH8SBQmyiGQqvh6NquhTnLoeg4KVbpZm5Hg=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=bWsUL/OoaLLpgmQL9nivhTPbiPR4rOVYhF3do48wrRRHNAc2cDtSECCmh9cLNs9iY7KN6qcXKYU+IhVV+hFM1z3z5D+UmE4UP7/q4pt1rmA5u1OLFUGJ4VgGyXCzP/RaNhbcbLgM3OED2iTXs9Bbb1CyJ0bp24MWPSu3e6RoeIs= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=er11rJpD; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="er11rJpD" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 16D1F1F000E9; Thu, 6 Aug 2026 17:32:28 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linuxfoundation.org; s=korg; t=1786037549; bh=R4ZFeVbJGt8JQXfi/s0pKE0fDRb0IkGlv/GwHhjHvtw=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=er11rJpDoY3sXLgnESJgeXHyGWV5w9HBC4z9jpM1V4vlzKqK3koM67NX/zDGPvkrC RUnwjbR+NhFjtMW5hiqvaHWZ1pNlq98IgTlOLjL3688vyXNfiFkTgFSx/dvueVY4SY CLr7C3lc3LdaWiwLNsAe7+NzAfx754NbIuFjp8ag= From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org, akpm@linux-foundation.org, torvalds@linux-foundation.org, stable@vger.kernel.org Cc: lwn@lwn.net, jslaby@suse.cz, Greg Kroah-Hartman Subject: Re: Linux 5.15.214 Date: Thu, 6 Aug 2026 19:32:05 +0200 Message-ID: <2026080605-unfiled-small-3f18@gregkh> X-Mailer: git-send-email 2.55.0 In-Reply-To: <2026080605-mullets-fetch-9e49@gregkh> References: <2026080605-mullets-fetch-9e49@gregkh> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit diff --git a/Makefile b/Makefile index 783acda54d74..6f062aab0a83 100644 --- a/Makefile +++ b/Makefile @@ -1,7 +1,7 @@ # SPDX-License-Identifier: GPL-2.0 VERSION = 5 PATCHLEVEL = 15 -SUBLEVEL = 213 +SUBLEVEL = 214 EXTRAVERSION = NAME = Trick or Treat diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S index ed74778c8ebd..5c5cb8ac6c14 100644 --- a/arch/x86/entry/entry_64.S +++ b/arch/x86/entry/entry_64.S @@ -941,6 +941,8 @@ SYM_CODE_START_LOCAL(paranoid_entry) IBRS_ENTER save_reg=%r15 UNTRAIN_RET + HANDLE_INTR_SAFERET 8(%rsp) + RET SYM_CODE_END(paranoid_entry) @@ -1045,6 +1047,9 @@ SYM_CODE_START_LOCAL(error_entry) movl %ecx, %eax /* zero extend */ cmpq %rax, RIP+8(%rsp) je .Lbstep_iret + + HANDLE_INTR_SAFERET 8(%rsp) + cmpq $.Lgs_change, RIP+8(%rsp) jne .Lerror_entry_done_lfence diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h index 0a366da54087..21c3610ff1d3 100644 --- a/arch/x86/include/asm/nospec-branch.h +++ b/arch/x86/include/asm/nospec-branch.h @@ -87,6 +87,50 @@ add $(BITS_PER_LONG/8), %_ASM_SP; \ lfence; +/* + * Helper for detecting if an interrupt occurred at an unsafe location within + * Safe-RET. If Safe-RET is interrupted after the CALL or LEA the RSB may get + * poisoned by the interrupt handler. + * + * The Safe-RET sequence is: + * + * CALL + * LEA 8(%RSP), %RSP + * RET + * + * The two CMPs below check whether RIP points to after the CALL or after the + * LEA. + * + * The LFENCE below is to address this particular speculation case: + * + * 1. Userspace runs and poisons the BTB around the safe-RET routine + * + * 2. Userspace triggers some kind of exception + * + * 3. Kernel executes error_entry() and mis-speculates the branch into thinking + * it actually came from kernel space + * + * 4. The kernel then further mis-speculates that the exception occurred due + * to an interrupted safe-RET + * + * 5. The handle_interrupted_saferet() routine speculatively executes and + * speculatively does a safe-RET. But this is unsafe since it was never + * untrained. + * + * The LFENCE fixes this by ensuring step 5 is never reached speculatively. + * Note that this LFENCE only occurs if safe-RET was actually interrupted (so + * it's outside of the normal path). + */ +#define __HANDLE_INTR_SAFERET(name, pt_regs) \ + cmpq $(name), RIP+pt_regs; \ + jb 1f; \ + cmpq $(name)+5, RIP+pt_regs; \ + ja 1f; \ + lfence; \ + leaq pt_regs, %rdi; \ + call handle_interrupted_saferet; \ + 1: + #ifdef __ASSEMBLY__ /* @@ -201,6 +245,14 @@ #endif .endm +.macro HANDLE_INTR_SAFERET pt_regs +#ifdef CONFIG_MITIGATION_SRSO + ALTERNATIVE_2 "", \ + __stringify(__HANDLE_INTR_SAFERET(srso_safe_ret, \pt_regs)), X86_FEATURE_SRSO, \ + __stringify(__HANDLE_INTR_SAFERET(srso_alias_safe_ret, \pt_regs)), X86_FEATURE_SRSO_ALIAS +#endif +.endm + /* * Macro to execute VERW insns that mitigate transient data sampling * attacks such as MDS or TSA. On affected systems a microcode update @@ -482,6 +534,10 @@ static __always_inline void x86_idle_clear_cpu_buffers(void) x86_clear_cpu_buffers(); } +void srso_safe_ret(void); +void srso_alias_safe_ret(void); +void handle_interrupted_saferet(struct pt_regs *regs); + #endif /* __ASSEMBLY__ */ #endif /* _ASM_X86_NOSPEC_BRANCH_H_ */ diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c index 8df48691f491..62ac691057ae 100644 --- a/arch/x86/kernel/cpu/bugs.c +++ b/arch/x86/kernel/cpu/bugs.c @@ -3397,3 +3397,42 @@ ssize_t cpu_show_vmscape(struct device *dev, struct device_attribute *attr, char return cpu_show_common(dev, attr, buf, X86_BUG_VMSCAPE); } #endif + +#ifdef CONFIG_MITIGATION_SRSO +/* + * Called during exception/interrupt entry if interrupted during the + * safe-RET sequence. The safe-RET sequence consists of 3 instructions: + * + * CALL + * LEA 8(%RSP), %RSP + * RET + * + * An interrupt after the CALL or after the LEA could potentially lead + * to branch predictor poisoning and results in the sequence not being + * able to be safely resumed. + * + * Therefore, modify the regs state as if the remaining part of the + * safe-RET sequence executed so the interrupt returns back to the + * desired return target, instead of the to the safe-RET sequence. + */ +void noinstr handle_interrupted_saferet(struct pt_regs *regs) +{ + unsigned long rip = regs->ip; + + if (rip == (unsigned long) srso_safe_ret || + rip == (unsigned long) srso_alias_safe_ret) { + /* Modify stack pointer as if LEA executed: */ + regs->sp += 8; + } + + /* + * Adjust registers as if RET executed: + * + * 1. Read the return address off the stack and into rIP: + */ + regs->ip = *(unsigned long *)(regs->sp); + + /* 2. Pop rIP off the stack: */ + regs->sp += 8; +} +#endif /* CONFIG_MITIGATION_SRSO */ diff --git a/arch/x86/lib/retpoline.S b/arch/x86/lib/retpoline.S index ae0151c6caba..6d9d83398cd8 100644 --- a/arch/x86/lib/retpoline.S +++ b/arch/x86/lib/retpoline.S @@ -112,10 +112,24 @@ __EXPORT_THUNK(srso_alias_untrain_ret) #endif SYM_START(srso_alias_safe_ret, SYM_L_GLOBAL, SYM_A_NONE) + + /* + * Tell objtool that those are not function pointers referenced by + * __HANDLE_INTR_SAFERET(). Below too. + */ + ANNOTATE_NOENDBR + + /* + * Safe-RET sequence. If you need to change it, adjust + * handle_interrupted_saferet() too. + */ lea 8(%_ASM_SP), %_ASM_SP UNWIND_HINT_FUNC + + ANNOTATE_NOENDBR ANNOTATE_UNRET_SAFE ret + /* End of Safe-RET sequence */ int3 SYM_FUNC_END(srso_alias_safe_ret) @@ -230,8 +244,14 @@ SYM_START(srso_untrain_ret, SYM_L_GLOBAL, SYM_A_NONE) * the stack. */ SYM_INNER_LABEL(srso_safe_ret, SYM_L_GLOBAL) + /* + * Safe-RET sequence. If you need to change it, adjust + * handle_interrupted_saferet() too. + */ lea 8(%_ASM_SP), %_ASM_SP ret + /* End of Safe-RET sequence */ + int3 int3 /* end of movabs */