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 1B69928850D for ; Sat, 4 Apr 2026 01:41:59 +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=1775266920; cv=none; b=A1dxg/111KwsjJyUcS9BgHad3pH9Tvv3J0DLfCRtWxumPfwITBUTntqmeqf/olfMnROalA98usisI9jMcTllUjx5QwbMmFZlChjdy7nDCZtlZvrt1cVGu2/LMOcnPgZ2ny68MznZR2xO83/R2VC6d+2jjlpq6B0RLfCRX4yxjEg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775266920; c=relaxed/simple; bh=t3tZJRto3dgjGBZ/8tjNyfgxbg71EB921DL6xq/1tCg=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version:Content-Type; b=L0PlsQ9U4Q7tgQzVF5+3tL3R4xFyMcCCab4n2hqhyxRosWqLnveXI0jDqpR8/0cJCayo/4zRSynmaq1s9KaR/9yUd+2pN7BojqqH3R1fQ9v96+IdayA2c9EY/hebjS3ZzhM3b8UdGBJ1e/7cMt9wnYuY3NgzLyEUWJnVVUw0kWA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=lVNJrgQH; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="lVNJrgQH" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 16C2EC4CEF7; Sat, 4 Apr 2026 01:41:59 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1775266919; bh=t3tZJRto3dgjGBZ/8tjNyfgxbg71EB921DL6xq/1tCg=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=lVNJrgQHamY65JrpJHmba3iMx6CH+ALB8z6t40u4UcepZNjiOCugYEBwRCwPai2Ce IV5ESNkpZX7vHlquR2a/ocLNyIXK+qdeuXX2jaVHBSCO4+GN/ozqFQSjti/ddDTpAu ZuGPqwdv77v/SCoBTvz6Oe3Y7/0biWxaw4QEMUeJKorIjdwS89qE8pgXAhVveXf4kT 2vixbz44HGuh84gUge47eLcKjy3026ZYc+HEb6hH7Pw/fzr+y7tbMI8itHZXCydFDw BIEduGyrtn5Yxud/MiivnDUTZOhl18TTBWbnH9pLt5fiwPiV4XEOWWV1jlpspBTTty h+wFVaM4FDFGA== From: Paul Walmsley To: linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org Cc: torvalds@linux-foundation.org, tglx@linutronix.de, peterz@infradead.org, broonie@kernel.org, Deepak Gupta Subject: [PATCH 6/6] prctl: cfi: change the branch landing pad prctl()s to be more descriptive Date: Fri, 3 Apr 2026 19:41:58 -0600 Message-ID: <177526691646.878386.5013965326452623846.stgit@seppa> X-Mailer: git-send-email 2.51.0 In-Reply-To: <177526680875.878386.16551569883089050686.stgit@seppa> References: <177526680875.878386.16551569883089050686.stgit@seppa> User-Agent: StGit/0.19 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 8bit Per Linus' comments requesting the replacement of "INDIR_BR_LP" in the indirect branch tracking prctl()s with something more readable, and suggesting the use of the speculation control prctl()s as an exemplar, reimplement the prctl()s and related constants that control per-task forward-edge control flow integrity. This primarily involves two changes. First, the prctls are restructured to resemble the style of the speculative execution workaround control prctls PR_{GET,SET}_SPECULATION_CTRL, to make them easier to extend in the future. Second, the "indir_br_lp" abbrevation is expanded to "branch_landing_pads" to be less telegraphic. The kselftest and documentation is adjusted accordingly. Link: https://lore.kernel.org/linux-riscv/CAHk-=whhSLGZAx3N5jJpb4GLFDqH_QvS07D+6BnkPWmCEzTAgw@mail.gmail.com/ Cc: Deepak Gupta Cc: Linus Torvalds Cc: Mark Brown Signed-off-by: Paul Walmsley --- Documentation/arch/riscv/zicfilp.rst | 63 ++++++++++++-------- arch/riscv/kernel/usercfi.c | 15 ++--- include/uapi/linux/prctl.h | 37 +++++------- kernel/sys.c | 23 ++++--- tools/perf/trace/beauty/include/uapi/linux/prctl.h | 36 +++++------ tools/testing/selftests/riscv/cfi/cfitests.c | 4 + 6 files changed, 91 insertions(+), 87 deletions(-) diff --git a/Documentation/arch/riscv/zicfilp.rst b/Documentation/arch/riscv/zicfilp.rst index 78a3e01ff68c..ab7d8e62ddaf 100644 --- a/Documentation/arch/riscv/zicfilp.rst +++ b/Documentation/arch/riscv/zicfilp.rst @@ -76,34 +76,49 @@ the program. 4. prctl() enabling -------------------- -:c:macro:`PR_SET_INDIR_BR_LP_STATUS` / :c:macro:`PR_GET_INDIR_BR_LP_STATUS` / -:c:macro:`PR_LOCK_INDIR_BR_LP_STATUS` are three prctls added to manage indirect -branch tracking. These prctls are architecture-agnostic and return -EINVAL if -the underlying functionality is not supported. +Per-task indirect branch tracking state can be monitored and +controlled via the :c:macro:`PR_GET_CFI` and :c:macro:`PR_SET_CFI` +``prctl()` arguments (respectively), by supplying +:c:macro:`PR_CFI_BRANCH_LANDING_PADS` as the second argument. These +are architecture-agnostic, and will return -EINVAL if the underlying +functionality is not supported. -* prctl(PR_SET_INDIR_BR_LP_STATUS, unsigned long arg) +* prctl(:c:macro:`PR_SET_CFI`, :c:macro:`PR_CFI_BRANCH_LANDING_PADS`, unsigned long arg) -If arg1 is :c:macro:`PR_INDIR_BR_LP_ENABLE` and if CPU supports -``zicfilp`` then the kernel will enable indirect branch tracking for the -task. The dynamic loader can issue this :c:macro:`prctl` once it has -determined that all the objects loaded in the address space support -indirect branch tracking. Additionally, if there is a `dlopen` to an -object which wasn't compiled with ``zicfilp``, the dynamic loader can -issue this prctl with arg1 set to 0 (i.e. :c:macro:`PR_INDIR_BR_LP_ENABLE` -cleared). - -* prctl(PR_GET_INDIR_BR_LP_STATUS, unsigned long * arg) +arg is a bitmask. -Returns the current status of indirect branch tracking. If enabled -it'll return :c:macro:`PR_INDIR_BR_LP_ENABLE` - -* prctl(PR_LOCK_INDIR_BR_LP_STATUS, unsigned long arg) +If :c:macro:`PR_CFI_ENABLE` is set in arg, and the CPU supports +``zicfilp``, then the kernel will enable indirect branch tracking for +the task. The dynamic loader can issue this ``prctl()`` once it has +determined that all the objects loaded in the address space support +indirect branch tracking. + +Indirect branch tracking state can also be locked once enabled. This +prevents the task from subsequently disabling it. This is done by +setting the bit :c:macro:`PR_CFI_LOCK` in arg. Either indirect branch +tracking must already be enabled for the task, or the bit +:c:macro:`PR_CFI_ENABLE` must also be set in arg. This is intended +for environments that wish to run with a strict security posture that +do not wish to load objects without ``zicfilp`` support. + +Indirect branch tracking can also be disabled for the task, assuming +that it has not previously been enabled and locked. If there is a +``dlopen()`` to an object which wasn't compiled with ``zicfilp``, the +dynamic loader can issue this ``prctl()`` with arg set to +:c:macro:`PR_CFI_DISABLE`. Disabling indirect branch tracking for the +task is not possible if it has previously been enabled and locked. + + +* prctl(:c:macro:`PR_GET_CFI`, :c:macro:`PR_CFI_BRANCH_LANDING_PADS`, unsigned long * arg) + +Returns the current status of indirect branch tracking into a bitmask +stored into the memory location pointed to by arg. The bitmask will +have the :c:macro:`PR_CFI_ENABLE` bit set if indirect branch tracking +is currently enabled for the task, and if it is locked, will +additionally have the :c:macro:`PR_CFI_LOCK` bit set. If indirect +branch tracking is currently disabled for the task, the +:c:macro:`PR_CFI_DISABLE` bit will be set. -Locks the current status of indirect branch tracking on the task. User -space may want to run with a strict security posture and wouldn't want -loading of objects without ``zicfilp`` support in them, to disallow -disabling of indirect branch tracking. In this case, user space can -use this prctl to lock the current settings. 5. violations related to indirect branch tracking -------------------------------------------------- diff --git a/arch/riscv/kernel/usercfi.c b/arch/riscv/kernel/usercfi.c index 04ab1eb8df29..2c535737511d 100644 --- a/arch/riscv/kernel/usercfi.c +++ b/arch/riscv/kernel/usercfi.c @@ -465,16 +465,14 @@ int arch_prctl_get_branch_landing_pad_state(struct task_struct *t, if (!is_user_lpad_enabled()) return -EINVAL; - /* indirect branch tracking is enabled on the task or not */ - fcfi_status |= (is_indir_lp_enabled(t) ? PR_INDIR_BR_LP_ENABLE : 0); + fcfi_status = (is_indir_lp_enabled(t) ? PR_CFI_ENABLE : PR_CFI_DISABLE); + fcfi_status |= (is_indir_lp_locked(t) ? PR_CFI_LOCK : 0); return copy_to_user(state, &fcfi_status, sizeof(fcfi_status)) ? -EFAULT : 0; } int arch_prctl_set_branch_landing_pad_state(struct task_struct *t, unsigned long state) { - bool enable_indir_lp = false; - if (!is_user_lpad_enabled()) return -EINVAL; @@ -482,12 +480,13 @@ int arch_prctl_set_branch_landing_pad_state(struct task_struct *t, unsigned long if (is_indir_lp_locked(t)) return -EINVAL; - /* Reject unknown flags */ - if (state & ~PR_INDIR_BR_LP_ENABLE) + if (!(state & (PR_CFI_ENABLE | PR_CFI_DISABLE))) + return -EINVAL; + + if (state & PR_CFI_ENABLE && state & PR_CFI_DISABLE) return -EINVAL; - enable_indir_lp = (state & PR_INDIR_BR_LP_ENABLE); - set_indir_lp_status(t, enable_indir_lp); + set_indir_lp_status(t, !!(state & PR_CFI_ENABLE)); return 0; } diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h index 55b0446fff9d..b6ec6f693719 100644 --- a/include/uapi/linux/prctl.h +++ b/include/uapi/linux/prctl.h @@ -397,30 +397,23 @@ struct prctl_mm_map { # define PR_RSEQ_SLICE_EXT_ENABLE 0x01 /* - * Get the current indirect branch tracking configuration for the current - * thread, this will be the value configured via PR_SET_INDIR_BR_LP_STATUS. + * Get or set the control flow integrity (CFI) configuration for the + * current thread. + * + * Some per-thread control flow integrity settings are not yet + * controlled through this prctl(); see for example + * PR_{GET,SET,LOCK}_SHADOW_STACK_STATUS */ -#define PR_GET_INDIR_BR_LP_STATUS 80 - +#define PR_GET_CFI 80 +#define PR_SET_CFI 81 /* - * Set the indirect branch tracking configuration. PR_INDIR_BR_LP_ENABLE will - * enable cpu feature for user thread, to track all indirect branches and ensure - * they land on arch defined landing pad instruction. - * x86 - If enabled, an indirect branch must land on an ENDBRANCH instruction. - * arch64 - If enabled, an indirect branch must land on a BTI instruction. - * riscv - If enabled, an indirect branch must land on an lpad instruction. - * PR_INDIR_BR_LP_DISABLE will disable feature for user thread and indirect - * branches will no more be tracked by cpu to land on arch defined landing pad - * instruction. - */ -#define PR_SET_INDIR_BR_LP_STATUS 81 -# define PR_INDIR_BR_LP_ENABLE (1UL << 0) - -/* - * Prevent further changes to the specified indirect branch tracking - * configuration. All bits may be locked via this call, including - * undefined bits. + * Forward-edge CFI variants (excluding ARM64 BTI, which has its own + * prctl()s). */ -#define PR_LOCK_INDIR_BR_LP_STATUS 82 +#define PR_CFI_BRANCH_LANDING_PADS 0 +/* Return and control values for PR_{GET,SET}_CFI */ +# define PR_CFI_ENABLE _BITUL(0) +# define PR_CFI_DISABLE _BITUL(1) +# define PR_CFI_LOCK _BITUL(2) #endif /* _LINUX_PRCTL_H */ diff --git a/kernel/sys.c b/kernel/sys.c index a5e0c187bbbf..62e842055cc9 100644 --- a/kernel/sys.c +++ b/kernel/sys.c @@ -2889,20 +2889,23 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3, return -EINVAL; error = rseq_slice_extension_prctl(arg2, arg3); break; - case PR_GET_INDIR_BR_LP_STATUS: - if (arg3 || arg4 || arg5) + case PR_GET_CFI: + if (arg2 != PR_CFI_BRANCH_LANDING_PADS) return -EINVAL; - error = arch_prctl_get_branch_landing_pad_state(me, (unsigned long __user *)arg2); - break; - case PR_SET_INDIR_BR_LP_STATUS: - if (arg3 || arg4 || arg5) + if (arg4 || arg5) return -EINVAL; - error = arch_prctl_set_branch_landing_pad_state(me, arg2); + error = arch_prctl_get_branch_landing_pad_state(me, (unsigned long __user *)arg3); break; - case PR_LOCK_INDIR_BR_LP_STATUS: - if (arg2 || arg3 || arg4 || arg5) + case PR_SET_CFI: + if (arg2 != PR_CFI_BRANCH_LANDING_PADS) return -EINVAL; - error = arch_prctl_lock_branch_landing_pad_state(me); + if (arg4 || arg5) + return -EINVAL; + error = arch_prctl_set_branch_landing_pad_state(me, arg3); + if (error) + break; + if (arg3 & PR_CFI_LOCK && !(arg3 & PR_CFI_DISABLE)) + error = arch_prctl_lock_branch_landing_pad_state(me); break; default: trace_task_prctl_unknown(option, arg2, arg3, arg4, arg5); diff --git a/tools/perf/trace/beauty/include/uapi/linux/prctl.h b/tools/perf/trace/beauty/include/uapi/linux/prctl.h index 55b0446fff9d..560f99bc4782 100644 --- a/tools/perf/trace/beauty/include/uapi/linux/prctl.h +++ b/tools/perf/trace/beauty/include/uapi/linux/prctl.h @@ -397,30 +397,24 @@ struct prctl_mm_map { # define PR_RSEQ_SLICE_EXT_ENABLE 0x01 /* - * Get the current indirect branch tracking configuration for the current - * thread, this will be the value configured via PR_SET_INDIR_BR_LP_STATUS. + * Get or set the control flow integrity (CFI) configuration for the + * current thread. + * + * Some per-thread control flow integrity settings are not yet + * controlled through this prctl(); see for example + * PR_{GET,SET,LOCK}_SHADOW_STACK_STATUS */ -#define PR_GET_INDIR_BR_LP_STATUS 80 - +#define PR_GET_CFI 80 +#define PR_SET_CFI 81 /* - * Set the indirect branch tracking configuration. PR_INDIR_BR_LP_ENABLE will - * enable cpu feature for user thread, to track all indirect branches and ensure - * they land on arch defined landing pad instruction. - * x86 - If enabled, an indirect branch must land on an ENDBRANCH instruction. - * arch64 - If enabled, an indirect branch must land on a BTI instruction. - * riscv - If enabled, an indirect branch must land on an lpad instruction. - * PR_INDIR_BR_LP_DISABLE will disable feature for user thread and indirect - * branches will no more be tracked by cpu to land on arch defined landing pad - * instruction. + * Forward-edge CFI variants (excluding ARM64 BTI, which has its own + * prctl()s). */ -#define PR_SET_INDIR_BR_LP_STATUS 81 -# define PR_INDIR_BR_LP_ENABLE (1UL << 0) +#define PR_CFI_BRANCH_LANDING_PADS 0 +/* Return and control values for PR_{GET,SET}_CFI */ +# define PR_CFI_ENABLE _BITUL(0) +# define PR_CFI_DISABLE _BITUL(1) +# define PR_CFI_LOCK _BITUL(2) -/* - * Prevent further changes to the specified indirect branch tracking - * configuration. All bits may be locked via this call, including - * undefined bits. - */ -#define PR_LOCK_INDIR_BR_LP_STATUS 82 #endif /* _LINUX_PRCTL_H */ diff --git a/tools/testing/selftests/riscv/cfi/cfitests.c b/tools/testing/selftests/riscv/cfi/cfitests.c index 0dac74b8553c..39d097b6881f 100644 --- a/tools/testing/selftests/riscv/cfi/cfitests.c +++ b/tools/testing/selftests/riscv/cfi/cfitests.c @@ -146,11 +146,11 @@ int main(int argc, char *argv[]) * pads for user mode except lighting up a bit in senvcfg via a prctl. * Enable landing pad support throughout the execution of the test binary. */ - ret = my_syscall5(__NR_prctl, PR_GET_INDIR_BR_LP_STATUS, &lpad_status, 0, 0, 0); + ret = my_syscall5(__NR_prctl, PR_GET_CFI, PR_CFI_BRANCH_LANDING_PADS, &lpad_status, 0, 0); if (ret) ksft_exit_fail_msg("Get landing pad status failed with %d\n", ret); - if (!(lpad_status & PR_INDIR_BR_LP_ENABLE)) + if (!(lpad_status & PR_CFI_ENABLE)) ksft_exit_fail_msg("Landing pad is not enabled, should be enabled via glibc\n"); ret = my_syscall5(__NR_prctl, PR_GET_SHADOW_STACK_STATUS, &ss_status, 0, 0, 0);