[PATCH v2 10/11] perf/uprobe: Convert single-step and uretprobe to SRCU

Peter Zijlstra
2024-07-11T11:07:29Z

Both single-step and uretprobes take a refcount on struct uprobe in
handle_swbp() in order to ensure struct uprobe stays extant until a
next trap.

Since uprobe_unregister() only cares about the uprobe_consumer
life-time, and these intra-trap sections can be arbitrarily large,
create a second SRCU domain to cover these.

Notably, a uretprobe with a registered return_instance that never
triggers -- because userspace -- will currently pin the
return_instance and related uprobe until the task dies. With this
convertion to SRCU this behaviour will inhibit freeing of all uprobes.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
include/linux/uprobes.h | 2 +
kernel/events/uprobes.c | 60 +++++++++++++++++++++++-------------------------
2 files changed, 31 insertions(+), 31 deletions(-)

--- a/include/linux/uprobes.h
+++ b/include/linux/uprobes.h
@@ -78,6 +78,7 @@ struct uprobe_task {

struct return_instance *return_instances;
unsigned int depth;
+ unsigned int active_srcu_idx;
};

struct return_instance {
@@ -86,6 +87,7 @@ struct return_instance {
unsigned long stack; /* stack pointer */
unsigned long orig_ret_vaddr; /* original return address */
bool chained; /* true, if instance is nested */
+ int srcu_idx;

struct return_instance *next; /* keep as stack */
};
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -54,6 +54,15 @@ DEFINE_STATIC_PERCPU_RWSEM(dup_mmap_sem)
* Covers uprobe->consumers lifetime as well as struct uprobe.
*/
DEFINE_STATIC_SRCU(uprobes_srcu);
+/*
+ * Covers return_instance->uprobe and utask->active_uprobe. Separate from
+ * uprobe_srcu because uprobe_unregister() doesn't need to wait for this
+ * and these lifetimes can be fairly long.
+ *
+ * Notably, these sections span userspace and as such use
+ * __srcu_read_{,un}lock() to elide lockdep.
+ */
+DEFINE_STATIC_SRCU(uretprobes_srcu);

/* Have a copy of original instruction */
#define UPROBE_COPY_INSN 0
@@ -596,25 +605,24 @@ set_orig_insn(struct arch_uprobe *auprob
*(uprobe_opcode_t *)&auprobe->insn);
}

-static struct uprobe *try_get_uprobe(struct uprobe *uprobe)
-{
- if (refcount_inc_not_zero(&uprobe->ref))
- return uprobe;
- return NULL;
-}
-
static struct uprobe *get_uprobe(struct uprobe *uprobe)
{
refcount_inc(&uprobe->ref);
return uprobe;
}

-static void uprobe_free_rcu(struct rcu_head *rcu)
+static void uprobe_free_stage2(struct rcu_head *rcu)
{
struct uprobe *uprobe = container_of(rcu, struct uprobe, rcu);
kfree(uprobe);
}

+static void uprobe_free_stage1(struct rcu_head *rcu)
+{
+ struct uprobe *uprobe = container_of(rcu, struct uprobe, rcu);
+ call_srcu(&uretprobes_srcu, &uprobe->rcu, uprobe_free_stage2);
+}
+
static void put_uprobe(struct uprobe *uprobe)
{
if (refcount_dec_and_test(&uprobe->ref)) {
@@ -626,7 +634,7 @@ static void put_uprobe(struct uprobe *up
mutex_lock(&delayed_uprobe_lock);
delayed_uprobe_remove(uprobe, NULL);
mutex_unlock(&delayed_uprobe_lock);
- call_srcu(&uprobes_srcu, &uprobe->rcu, uprobe_free_rcu);
+ call_srcu(&uprobes_srcu, &uprobe->rcu, uprobe_free_stage1);
}
}

@@ -1753,7 +1761,7 @@ unsigned long uprobe_get_trap_addr(struc
static struct return_instance *free_ret_instance(struct return_instance *ri)
{
struct return_instance *next = ri->next;
- put_uprobe(ri->uprobe);
+ __srcu_read_unlock(&uretprobes_srcu, ri->srcu_idx);
kfree(ri);
return next;
}
@@ -1771,7 +1779,7 @@ void uprobe_free_utask(struct task_struc
return;

if (utask->active_uprobe)
- put_uprobe(utask->active_uprobe);
+ __srcu_read_unlock(&uretprobes_srcu, utask->active_srcu_idx);

ri = utask->return_instances;
while (ri)
@@ -1814,7 +1822,7 @@ static int dup_utask(struct task_struct
return -ENOMEM;

*n = *o;
- get_uprobe(n->uprobe);
+ __srcu_clone_read_lock(&uretprobes_srcu, n->srcu_idx);
n->next = NULL;

*p = n;
@@ -1931,14 +1939,10 @@ static void prepare_uretprobe(struct upr
if (!ri)
return;

- ri->uprobe = try_get_uprobe(uprobe);
- if (!ri->uprobe)
- goto err_mem;
-
trampoline_vaddr = get_trampoline_vaddr();
orig_ret_vaddr = arch_uretprobe_hijack_return_addr(trampoline_vaddr, regs);
if (orig_ret_vaddr == -1)
- goto err_uprobe;
+ goto err_mem;

/* drop the entries invalidated by longjmp() */
chained = (orig_ret_vaddr == trampoline_vaddr);
@@ -1956,11 +1960,13 @@ static void prepare_uretprobe(struct upr
* attack from user-space.
*/
uprobe_warn(current, "handle tail call");
- goto err_uprobe;
+ goto err_mem;
}
orig_ret_vaddr = utask->return_instances->orig_ret_vaddr;
}

+ ri->srcu_idx = __srcu_read_lock(&uretprobes_srcu);
+ ri->uprobe = uprobe;
ri->func = instruction_pointer(regs);
ri->stack = user_stack_pointer(regs);
ri->orig_ret_vaddr = orig_ret_vaddr;
@@ -1972,8 +1978,6 @@ static void prepare_uretprobe(struct upr

return;

-err_uprobe:
- uprobe_put(ri->uprobe);
err_mem:
kfree(ri);
}
@@ -1990,15 +1994,9 @@ pre_ssout(struct uprobe *uprobe, struct
if (!utask)
return -ENOMEM;

- utask->active_uprobe = try_get_uprobe(uprobe);
- if (!utask->active_uprobe)
- return -ESRCH;
-
xol_vaddr = xol_get_insn_slot(uprobe);
- if (!xol_vaddr) {
- err = -ENOMEM;
- goto err_uprobe;
- }
+ if (!xol_vaddr)
+ return -ENOMEM;

utask->xol_vaddr = xol_vaddr;
utask->vaddr = bp_vaddr;
@@ -2007,13 +2005,13 @@ pre_ssout(struct uprobe *uprobe, struct
if (unlikely(err))
goto err_xol;

+ utask->active_srcu_idx = __srcu_read_lock(&uretprobes_srcu);
+ utask->active_uprobe = uprobe;
utask->state = UTASK_SSTEP;
return 0;

err_xol:
xol_free_insn_slot(current);
-err_uprobe:
- put_uprobe(utask->active_uprobe);
return err;
}

@@ -2366,7 +2364,7 @@ static void handle_singlestep(struct upr
else
WARN_ON_ONCE(1);

- put_uprobe(uprobe);
+ __srcu_read_unlock(&uretprobes_srcu, utask->active_srcu_idx);
utask->active_uprobe = NULL;
utask->state = UTASK_RUNNING;
xol_free_insn_slot(current);