Re: [patch V4 19/36] rseq: Provide and use rseq_update_user_cs()

Mathieu Desnoyers
2025-09-09T15:11:28Z

On 2025-09-08 17:32, Thomas Gleixner wrote:
> Provide a straight forward implementation to check for and eventually
> clear/fixup critical sections in user space.
>
> The non-debug version does only the minimal sanity checks and aims for
> efficiency.
>
> There are two attack vectors, which are checked for:
>
> 1) An abort IP which is in the kernel address space. That would cause at
> least x86 to return to kernel space via IRET.
>
> 2) A rogue critical section descriptor with an abort IP pointing to some
> arbitrary address, which is not preceeded by the RSEQ signature.

preceeded -> preceded

>
> If the section descriptors are invalid then the resulting misbehaviour of
> the user space application is not the kernels problem.
>
> The kernel provides a run-time switchable debug slow path, which implements
> the full zoo of checks including termination of the task when one of the
> gazillion conditions is not met.
>
> Replace the zoo in rseq.c with it and invoke it from the TIF_NOTIFY_RESUME
> handler. Move the reminders into the CONFIG_DEBUG_RSEQ section, which will

reminders -> remainders

> be replaced and removed in a subsequent step.
>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> ---
> V3: Brought back the signature check along with a comment - Mathieu
> ---
> include/linux/rseq_entry.h | 209 +++++++++++++++++++++++++++++++++++++++
> include/linux/rseq_types.h | 11 +-
> kernel/rseq.c | 238 +++++++++++++--------------------------------
> 3 files changed, 288 insertions(+), 170 deletions(-)
>
> --- a/include/linux/rseq_entry.h
> +++ b/include/linux/rseq_entry.h
> @@ -36,6 +36,7 @@ DECLARE_PER_CPU(struct rseq_stats, rseq_
> #ifdef CONFIG_RSEQ
> #include <linux/jump_label.h>
> #include <linux/rseq.h>
> +#include <linux/uaccess.h>
>
> #include <linux/tracepoint-defs.h>
>
> @@ -67,12 +68,220 @@ static inline void rseq_trace_ip_fixup(u
>
> DECLARE_STATIC_KEY_MAYBE(CONFIG_RSEQ_DEBUG_DEFAULT_ENABLE, rseq_debug_enabled);
>
> +#ifdef RSEQ_BUILD_SLOW_PATH
> +#define rseq_inline
> +#else
> +#define rseq_inline __always_inline
> +#endif
> +
> +bool rseq_debug_update_user_cs(struct task_struct *t, struct pt_regs *regs, unsigned long csaddr);
> +
> static __always_inline void rseq_note_user_irq_entry(void)
> {
> if (IS_ENABLED(CONFIG_GENERIC_IRQ_ENTRY))
> current->rseq.event.user_irq = true;
> }
>
> +/*
> + * Check whether there is a valid critical section and whether the
> + * instruction pointer in @regs is inside the critical section.
> + *
> + * - If the critical section is invalid, terminate the task.
> + *
> + * - If valid and the instruction pointer is inside, set it to the abort IP
> + *
> + * - If valid and the instruction pointer is outside, clear the critical
> + * section address.
> + *
> + * Returns true, if the section was valid and either fixup or clear was
> + * done, false otherwise.
> + *
> + * In the failure case task::rseq_event::fatal is set when a invalid
> + * section was found. It's clear when the failure was an unresolved page
> + * fault.
> + *
> + * If inlined into the exit to user path with interrupts disabled, the
> + * caller has to protect against page faults with pagefault_disable().
> + *
> + * In preemptible task context this would be counterproductive as the page
> + * faults could not be fully resolved. As a consequence unresolved page
> + * faults in task context are fatal too.
> + */
> +
> +#ifdef RSEQ_BUILD_SLOW_PATH
> +/*
> + * The debug version is put out of line, but kept here so the code stays
> + * together.
> + *
> + * @csaddr has already been checked by the caller to be in user space
> + */
> +bool rseq_debug_update_user_cs(struct task_struct *t, struct pt_regs *regs, unsigned long csaddr)
> +{
> + struct rseq_cs __user *ucs = (struct rseq_cs __user *)(unsigned long)csaddr;
> + u64 start_ip, abort_ip, offset, cs_end, head, tasksize = TASK_SIZE;
> + unsigned long ip = instruction_pointer(regs);
> + u64 __user *uc_head = (u64 __user *) ucs;
> + u32 usig, __user *uc_sig;
> +
> + if (!user_rw_masked_begin(ucs))
> + return false;
> +
> + /*
> + * Evaluate the user pile and exit if one of the conditions is not
> + * fulfilled.
> + */
> + unsafe_get_user(start_ip, &ucs->start_ip, fail);
> + if (unlikely(start_ip >= tasksize))
> + goto die;
> + /* If outside, just clear the critical section. */
> + if (ip < start_ip)
> + goto clear;
> +
> + unsafe_get_user(offset, &ucs->post_commit_offset, fail);
> + cs_end = start_ip + offset;
> + /* Check for overflow and wraparound */
> + if (unlikely(cs_end >= tasksize || cs_end < start_ip))
> + goto die;
> +
> + /* If not inside, clear it. */
> + if (ip >= cs_end)
> + goto clear;
> +
> + unsafe_get_user(abort_ip, &ucs->abort_ip, fail);
> + /* Ensure it's "valid" */
> + if (unlikely(abort_ip >= tasksize || abort_ip < sizeof(*uc_sig)))
> + goto die;
> + /* Validate that the abort IP is not in the critical section */
> + if (unlikely(abort_ip - start_ip < offset))
> + goto die;
> +
> + /*
> + * Check version and flags for 0. No point in emitting deprecated
> + * warnings before dying. That could be done in the slow path
> + * eventually, but *shrug*.
> + */
> + unsafe_get_user(head, uc_head, fail);
> + if (unlikely(head))
> + goto die;
> +
> + /* abort_ip - 4 is >= 0. See abort_ip check above */
> + uc_sig = (u32 __user *)(unsigned long)(abort_ip - sizeof(*uc_sig));
> + unsafe_get_user(usig, uc_sig, fail);
> + if (unlikely(usig != t->rseq.sig))
> + goto die;
> +
> + /* rseq_event.user_irq is only valid if CONFIG_GENERIC_IRQ_ENTRY=y */
> + if (IS_ENABLED(CONFIG_GENERIC_IRQ_ENTRY)) {
> + /* If not in interrupt from user context, let it die */
> + if (unlikely(!t->rseq.event.user_irq))
> + goto die;
> + }
> +
> + unsafe_put_user(0ULL, &t->rseq.usrptr->rseq_cs, fail);
> + user_access_end();
> +
> + instruction_pointer_set(regs, (unsigned long)abort_ip);
> +
> + rseq_stat_inc(rseq_stats.fixup);
> + rseq_trace_ip_fixup(ip, start_ip, offset, abort_ip);
> + return true;
> +clear:
> + unsafe_put_user(0ULL, &t->rseq.usrptr->rseq_cs, fail);
> + user_access_end();
> + rseq_stat_inc(rseq_stats.clear);
> + return true;
> +die:
> + t->rseq.event.fatal = true;
> +fail:
> + user_access_end();
> + return false;
> +}
> +#endif /* RSEQ_BUILD_SLOW_PATH */
> +
> +/*
> + * This only ensures that abort_ip is in the user address space by masking it.
> + * No other sanity checks are done here, that's what the debug code is for.

This comment should be updated to state that the signature is also
checked.

> + */
> +static rseq_inline bool
> +rseq_update_user_cs(struct task_struct *t, struct pt_regs *regs, unsigned long csaddr)
> +{
> + struct rseq_cs __user *ucs = (struct rseq_cs __user *)(unsigned long)csaddr;
> + unsigned long ip = instruction_pointer(regs);
> + u64 start_ip, abort_ip, offset;
> + u32 usig, __user *uc_sig;
> +
> + rseq_stat_inc(rseq_stats.cs);
> +
> + if (unlikely(csaddr >= TASK_SIZE)) {
> + t->rseq.event.fatal = true;
> + return false;
> + }
> +
> + if (static_branch_unlikely(&rseq_debug_enabled))
> + return rseq_debug_update_user_cs(t, regs, csaddr);
> +
> + if (!user_rw_masked_begin(ucs))
> + return false;
> +
> + unsafe_get_user(start_ip, &ucs->start_ip, fail);
> + unsafe_get_user(offset, &ucs->post_commit_offset, fail);
> + unsafe_get_user(abort_ip, &ucs->abort_ip, fail);
> +
> + /*
> + * No sanity checks. If user space screwed it up, it can
> + * keep the pieces. That's what debug code is for.
> + *
> + * If outside, just clear the critical section.
> + */
> + if (ip - start_ip >= offset)
> + goto clear;
> +
> + /*
> + * Two requirements for @abort_ip:
> + * - Must be in user space as x86 IRET would happily return to
> + * the kernel.
> + * - The four bytes preceeding the instruction at @abort_ip must

preceeding -> preceding

> + * contain the signature.
> + *
> + * The latter protects against the following attack vector:
> + *
> + * An attacker with limited abilities to write, creates a critical
> + * section descriptor, sets the abort IP to a library function or
> + * some other ROP gadget and stores the address of the descriptor
> + * in TLS::rseq::rseq_cs. An RSEQ abort would then evade ROP
> + * protection.

Thanks for documenting this.

Mathieu

> + */
> + if (abort_ip >= TASK_SIZE || abort_ip < sizeof(*uc_sig))
> + goto die;
> +
> + /* The address is guaranteed to be >= 0 and < TASK_SIZE */
> + uc_sig = (u32 __user *)(unsigned long)(abort_ip - sizeof(*uc_sig));
> + unsafe_get_user(usig, uc_sig, fail);
> + if (unlikely(usig != t->rseq.sig))
> + goto die;
> +
> + /* Invalidate the critical section */
> + unsafe_put_user(0ULL, &t->rseq.usrptr->rseq_cs, fail);
> + user_access_end();
> +
> + /* Update the instruction pointer */
> + instruction_pointer_set(regs, (unsigned long)abort_ip);
> +
> + rseq_stat_inc(rseq_stats.fixup);
> + rseq_trace_ip_fixup(ip, start_ip, offset, abort_ip);
> + return true;
> +clear:
> + unsafe_put_user(0ULL, &t->rseq.usrptr->rseq_cs, fail);
> + user_access_end();
> + rseq_stat_inc(rseq_stats.clear);
> + return true;
> +die:
> + t->rseq.event.fatal = true;
> +fail:
> + user_access_end();
> + return false;
> +}
> +
> static __always_inline void rseq_exit_to_user_mode(void)
> {
> struct rseq_event *ev = &current->rseq.event;
> --- a/include/linux/rseq_types.h
> +++ b/include/linux/rseq_types.h
> @@ -14,10 +14,12 @@ struct rseq;
> * @sched_switch: True if the task was scheduled out
> * @user_irq: True on interrupt entry from user mode
> * @has_rseq: True if the task has a rseq pointer installed
> + * @error: Compound error code for the slow path to analyze
> + * @fatal: User space data corrupted or invalid
> */
> struct rseq_event {
> union {
> - u32 all;
> + u64 all;
> struct {
> union {
> u16 events;
> @@ -28,6 +30,13 @@ struct rseq_event {
> };
>
> u8 has_rseq;
> + u8 __pad;
> + union {
> + u16 error;
> + struct {
> + u8 fatal;
> + };
> + };
> };
> };
> };
> --- a/kernel/rseq.c
> +++ b/kernel/rseq.c
> @@ -382,175 +382,15 @@ static int rseq_reset_rseq_cpu_node_id(s
> return -EFAULT;
> }
>
> -/*
> - * Get the user-space pointer value stored in the 'rseq_cs' field.
> - */
> -static int rseq_get_rseq_cs_ptr_val(struct rseq __user *rseq, u64 *rseq_cs)
> -{
> - if (!rseq_cs)
> - return -EFAULT;
> -
> -#ifdef CONFIG_64BIT
> - if (get_user(*rseq_cs, &rseq->rseq_cs))
> - return -EFAULT;
> -#else
> - if (copy_from_user(rseq_cs, &rseq->rseq_cs, sizeof(*rseq_cs)))
> - return -EFAULT;
> -#endif
> -
> - return 0;
> -}
> -
> -/*
> - * If the rseq_cs field of 'struct rseq' contains a valid pointer to
> - * user-space, copy 'struct rseq_cs' from user-space and validate its fields.
> - */
> -static int rseq_get_rseq_cs(struct task_struct *t, struct rseq_cs *rseq_cs)
> -{
> - struct rseq_cs __user *urseq_cs;
> - u64 ptr;
> - u32 __user *usig;
> - u32 sig;
> - int ret;
> -
> - ret = rseq_get_rseq_cs_ptr_val(t->rseq.usrptr, &ptr);
> - if (ret)
> - return ret;
> -
> - /* If the rseq_cs pointer is NULL, return a cleared struct rseq_cs. */
> - if (!ptr) {
> - memset(rseq_cs, 0, sizeof(*rseq_cs));
> - return 0;
> - }
> - /* Check that the pointer value fits in the user-space process space. */
> - if (ptr >= TASK_SIZE)
> - return -EINVAL;
> - urseq_cs = (struct rseq_cs __user *)(unsigned long)ptr;
> - if (copy_from_user(rseq_cs, urseq_cs, sizeof(*rseq_cs)))
> - return -EFAULT;
> -
> - if (rseq_cs->start_ip >= TASK_SIZE ||
> - rseq_cs->start_ip + rseq_cs->post_commit_offset >= TASK_SIZE ||
> - rseq_cs->abort_ip >= TASK_SIZE ||
> - rseq_cs->version > 0)
> - return -EINVAL;
> - /* Check for overflow. */
> - if (rseq_cs->start_ip + rseq_cs->post_commit_offset < rseq_cs->start_ip)
> - return -EINVAL;
> - /* Ensure that abort_ip is not in the critical section. */
> - if (rseq_cs->abort_ip - rseq_cs->start_ip < rseq_cs->post_commit_offset)
> - return -EINVAL;
> -
> - usig = (u32 __user *)(unsigned long)(rseq_cs->abort_ip - sizeof(u32));
> - ret = get_user(sig, usig);
> - if (ret)
> - return ret;
> -
> - if (current->rseq.sig != sig) {
> - printk_ratelimited(KERN_WARNING
> - "Possible attack attempt. Unexpected rseq signature 0x%x, expecting 0x%x (pid=%d, addr=%p).\n",
> - sig, current->rseq.sig, current->pid, usig);
> - return -EINVAL;
> - }
> - return 0;
> -}
> -
> -static bool rseq_warn_flags(const char *str, u32 flags)
> +static bool rseq_handle_cs(struct task_struct *t, struct pt_regs *regs)
> {
> - u32 test_flags;
> + u64 csaddr;
>
> - if (!flags)
> + if (get_user_masked_u64(&csaddr, &t->rseq.usrptr->rseq_cs))
> return false;
> - test_flags = flags & RSEQ_CS_NO_RESTART_FLAGS;
> - if (test_flags)
> - pr_warn_once("Deprecated flags (%u) in %s ABI structure", test_flags, str);
> - test_flags = flags & ~RSEQ_CS_NO_RESTART_FLAGS;
> - if (test_flags)
> - pr_warn_once("Unknown flags (%u) in %s ABI structure", test_flags, str);
> - return true;
> -}
> -
> -static int rseq_check_flags(struct task_struct *t, u32 cs_flags)
> -{
> - u32 flags;
> - int ret;
> -
> - if (rseq_warn_flags("rseq_cs", cs_flags))
> - return -EINVAL;
> -
> - /* Get thread flags. */
> - ret = get_user(flags, &t->rseq.usrptr->flags);
> - if (ret)
> - return ret;
> -
> - if (rseq_warn_flags("rseq", flags))
> - return -EINVAL;
> - return 0;
> -}
> -
> -static int clear_rseq_cs(struct rseq __user *rseq)
> -{
> - /*
> - * The rseq_cs field is set to NULL on preemption or signal
> - * delivery on top of rseq assembly block, as well as on top
> - * of code outside of the rseq assembly block. This performs
> - * a lazy clear of the rseq_cs field.
> - *
> - * Set rseq_cs to NULL.
> - */
> -#ifdef CONFIG_64BIT
> - return put_user(0UL, &rseq->rseq_cs);
> -#else
> - if (clear_user(&rseq->rseq_cs, sizeof(rseq->rseq_cs)))
> - return -EFAULT;
> - return 0;
> -#endif
> -}
> -
> -/*
> - * Unsigned comparison will be true when ip >= start_ip, and when
> - * ip < start_ip + post_commit_offset.
> - */
> -static bool in_rseq_cs(unsigned long ip, struct rseq_cs *rseq_cs)
> -{
> - return ip - rseq_cs->start_ip < rseq_cs->post_commit_offset;
> -}
> -
> -static int rseq_ip_fixup(struct pt_regs *regs, bool abort)
> -{
> - unsigned long ip = instruction_pointer(regs);
> - struct task_struct *t = current;
> - struct rseq_cs rseq_cs;
> - int ret;
> -
> - rseq_stat_inc(rseq_stats.cs);
> -
> - ret = rseq_get_rseq_cs(t, &rseq_cs);
> - if (ret)
> - return ret;
> -
> - /*
> - * Handle potentially not being within a critical section.
> - * If not nested over a rseq critical section, restart is useless.
> - * Clear the rseq_cs pointer and return.
> - */
> - if (!in_rseq_cs(ip, &rseq_cs)) {
> - rseq_stat_inc(rseq_stats.clear);
> - return clear_rseq_cs(t->rseq.usrptr);
> - }
> - ret = rseq_check_flags(t, rseq_cs.flags);
> - if (ret < 0)
> - return ret;
> - if (!abort)
> - return 0;
> - ret = clear_rseq_cs(t->rseq.usrptr);
> - if (ret)
> - return ret;
> - rseq_stat_inc(rseq_stats.fixup);
> - trace_rseq_ip_fixup(ip, rseq_cs.start_ip, rseq_cs.post_commit_offset,
> - rseq_cs.abort_ip);
> - instruction_pointer_set(regs, (unsigned long)rseq_cs.abort_ip);
> - return 0;
> + if (likely(!csaddr))
> + return true;
> + return rseq_update_user_cs(t, regs, csaddr);
> }
>
> /*
> @@ -567,8 +407,8 @@ static int rseq_ip_fixup(struct pt_regs
> void __rseq_handle_notify_resume(struct ksignal *ksig, struct pt_regs *regs)
> {
> struct task_struct *t = current;
> - int ret, sig;
> bool event;
> + int sig;
>
> /*
> * If invoked from hypervisors before entering the guest via
> @@ -618,8 +458,7 @@ void __rseq_handle_notify_resume(struct
> if (!IS_ENABLED(CONFIG_DEBUG_RSEQ) && !event)
> return;
>
> - ret = rseq_ip_fixup(regs, event);
> - if (unlikely(ret < 0))
> + if (!rseq_handle_cs(t, regs))
> goto error;
>
> if (unlikely(rseq_update_cpu_node_id(t)))
> @@ -632,6 +471,67 @@ void __rseq_handle_notify_resume(struct
> }
>
> #ifdef CONFIG_DEBUG_RSEQ
> +/*
> + * Unsigned comparison will be true when ip >= start_ip, and when
> + * ip < start_ip + post_commit_offset.
> + */
> +static bool in_rseq_cs(unsigned long ip, struct rseq_cs *rseq_cs)
> +{
> + return ip - rseq_cs->start_ip < rseq_cs->post_commit_offset;
> +}
> +
> +/*
> + * If the rseq_cs field of 'struct rseq' contains a valid pointer to
> + * user-space, copy 'struct rseq_cs' from user-space and validate its fields.
> + */
> +static int rseq_get_rseq_cs(struct task_struct *t, struct rseq_cs *rseq_cs)
> +{
> + struct rseq_cs __user *urseq_cs;
> + u64 ptr;
> + u32 __user *usig;
> + u32 sig;
> + int ret;
> +
> + if (get_user_masked_u64(&ptr, &t->rseq.usrptr->rseq_cs))
> + return -EFAULT;
> +
> + /* If the rseq_cs pointer is NULL, return a cleared struct rseq_cs. */
> + if (!ptr) {
> + memset(rseq_cs, 0, sizeof(*rseq_cs));
> + return 0;
> + }
> + /* Check that the pointer value fits in the user-space process space. */
> + if (ptr >= TASK_SIZE)
> + return -EINVAL;
> + urseq_cs = (struct rseq_cs __user *)(unsigned long)ptr;
> + if (copy_from_user(rseq_cs, urseq_cs, sizeof(*rseq_cs)))
> + return -EFAULT;
> +
> + if (rseq_cs->start_ip >= TASK_SIZE ||
> + rseq_cs->start_ip + rseq_cs->post_commit_offset >= TASK_SIZE ||
> + rseq_cs->abort_ip >= TASK_SIZE ||
> + rseq_cs->version > 0)
> + return -EINVAL;
> + /* Check for overflow. */
> + if (rseq_cs->start_ip + rseq_cs->post_commit_offset < rseq_cs->start_ip)
> + return -EINVAL;
> + /* Ensure that abort_ip is not in the critical section. */
> + if (rseq_cs->abort_ip - rseq_cs->start_ip < rseq_cs->post_commit_offset)
> + return -EINVAL;
> +
> + usig = (u32 __user *)(unsigned long)(rseq_cs->abort_ip - sizeof(u32));
> + ret = get_user(sig, usig);
> + if (ret)
> + return ret;
> +
> + if (current->rseq.sig != sig) {
> + printk_ratelimited(KERN_WARNING
> + "Possible attack attempt. Unexpected rseq signature 0x%x, expecting 0x%x (pid=%d, addr=%p).\n",
> + sig, current->rseq.sig, current->pid, usig);
> + return -EINVAL;
> + }
> + return 0;
> +}
>
> /*
> * Terminate the process if a syscall is issued within a restartable
>

--
Mathieu Desnoyers
EfficiOS Inc.