[RFC PATCH 2/4] rseq: Allow extending struct rseq
From: Mathieu Desnoyers
Date: Mon Jul 13 2020 - 23:04:01 EST
Add a __rseq_abi.flags "RSEQ_TLS_FLAG_SIZE", which indicates support for
extending struct rseq. This adds two new fields to struct rseq:
user_size and kernel_size.
The user_size field allows the size of the __rseq_abi definition (which
can be overridden by symbol interposition either by a preloaded library
or by the application) to be handed over to the kernel at registration.
This registration can be performed by a library, e.g. glibc, which does
not know there is interposition taking place.
The kernel_size is populated by the kernel when the "RSEQ_TLS_FLAG_SIZE"
flag is set in __rseq_abi.flags to the minimum between user_size and
the offset of the "end" field of struct rseq as known by the kernel.
This allows user-space to query which fields are effectively populated
by the kernel.
A rseq_size field is added to the task struct to keep track of the
"kernel_size" effective for each thread.
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
---
include/linux/sched.h | 4 ++++
include/uapi/linux/rseq.h | 37 ++++++++++++++++++++++++++++++++--
kernel/rseq.c | 42 +++++++++++++++++++++++++++++++++------
3 files changed, 75 insertions(+), 8 deletions(-)
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 692e327d7455..5d61a3197987 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1147,6 +1147,7 @@ struct task_struct {
#ifdef CONFIG_RSEQ
struct rseq __user *rseq;
u32 rseq_sig;
+ u32 rseq_size;
/*
* RmW on rseq_event_mask must be performed atomically
* with respect to preemption.
@@ -1976,10 +1977,12 @@ static inline void rseq_fork(struct task_struct *t, unsigned long clone_flags)
if (clone_flags & CLONE_VM) {
t->rseq = NULL;
t->rseq_sig = 0;
+ t->rseq_size = 0;
t->rseq_event_mask = 0;
} else {
t->rseq = current->rseq;
t->rseq_sig = current->rseq_sig;
+ t->rseq_size = current->rseq_size;
t->rseq_event_mask = current->rseq_event_mask;
}
}
@@ -1988,6 +1991,7 @@ static inline void rseq_execve(struct task_struct *t)
{
t->rseq = NULL;
t->rseq_sig = 0;
+ t->rseq_size = 0;
t->rseq_event_mask = 0;
}
diff --git a/include/uapi/linux/rseq.h b/include/uapi/linux/rseq.h
index e11d9df5e564..03c0b5e9a859 100644
--- a/include/uapi/linux/rseq.h
+++ b/include/uapi/linux/rseq.h
@@ -37,6 +37,15 @@ enum rseq_cs_flags {
(1U << RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE_BIT),
};
+enum rseq_tls_flags_bit {
+ /* enum rseq_cs_flags reserves bits 0-2. */
+ RSEQ_TLS_FLAG_SIZE_BIT = 3,
+};
+
+enum rseq_tls_flags {
+ RSEQ_TLS_FLAG_SIZE = (1U << RSEQ_TLS_FLAG_SIZE_BIT),
+};
+
/* The rseq_len expected by rseq registration is always 32 bytes. */
enum rseq_len_expected {
RSEQ_LEN_EXPECTED = 32,
@@ -133,8 +142,9 @@ struct rseq {
*
* This field should only be updated by the thread which
* registered this data structure. Read by the kernel.
- * Mainly used for single-stepping through rseq critical sections
- * with debuggers.
+ *
+ * The RSEQ_CS flags are mainly used for single-stepping through rseq
+ * critical sections with debuggers.
*
* - RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT
* Inhibit instruction sequence block restart on preemption
@@ -145,8 +155,31 @@ struct rseq {
* - RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE
* Inhibit instruction sequence block restart on migration for
* this thread.
+ *
+ * - RSEQ_TLS_FLAG_SIZE
+ * Extensible struct rseq ABI. This flag should be statically
+ * initialized.
*/
__u32 flags;
+ /*
+ * With __rseq_abi.flags RSEQ_TLS_FLAG_SIZE set, user_size should be
+ * statically initialized to offsetof(struct rseq, end).
+ */
+ __u16 user_size;
+ /*
+ * With __rseq_abi.flags RSEQ_TLS_FLAG_SIZE set, if the kernel supports
+ * extensible struct rseq ABI, the kernel_size field is populated by
+ * the kernel to the minimum between user_size and the offset of the
+ * "end" field within the struct rseq supported by the kernel on
+ * successful registration. Should be initialized to 0.
+ */
+ __u16 kernel_size;
+
+ /*
+ * Very last field of the structure, to calculate size excluding padding
+ * with offsetof().
+ */
+ char end[];
} __attribute__((aligned(4 * sizeof(__u64))));
#endif /* _UAPI_LINUX_RSEQ_H */
diff --git a/kernel/rseq.c b/kernel/rseq.c
index a4f86a9d6937..bbc57fc18573 100644
--- a/kernel/rseq.c
+++ b/kernel/rseq.c
@@ -96,6 +96,7 @@ static int rseq_update_cpu_id(struct task_struct *t)
static int rseq_reset_rseq_cpu_id(struct task_struct *t)
{
u32 cpu_id_start = 0, cpu_id = RSEQ_CPU_ID_UNINITIALIZED;
+ u16 kernel_size = 0;
/*
* Reset cpu_id_start to its initial state (0).
@@ -109,6 +110,11 @@ static int rseq_reset_rseq_cpu_id(struct task_struct *t)
*/
if (put_user(cpu_id, &t->rseq->cpu_id))
return -EFAULT;
+ /*
+ * Reset kernel_size to its initial state (0).
+ */
+ if (put_user(kernel_size, &t->rseq->kernel_size))
+ return -EFAULT;
return 0;
}
@@ -266,7 +272,7 @@ void __rseq_handle_notify_resume(struct ksignal *ksig, struct pt_regs *regs)
if (unlikely(t->flags & PF_EXITING))
return;
- if (unlikely(!access_ok(t->rseq, sizeof(*t->rseq))))
+ if (unlikely(!access_ok(t->rseq, t->rseq_size)))
goto error;
ret = rseq_ip_fixup(regs);
if (unlikely(ret < 0))
@@ -294,7 +300,7 @@ void rseq_syscall(struct pt_regs *regs)
if (!t->rseq)
return;
- if (!access_ok(t->rseq, sizeof(*t->rseq)) ||
+ if (!access_ok(t->rseq, t->rseq_size) ||
rseq_get_rseq_cs(t, &rseq_cs) || in_rseq_cs(ip, &rseq_cs))
force_sig(SIGSEGV);
}
@@ -308,6 +314,7 @@ SYSCALL_DEFINE4(rseq, struct rseq __user *, rseq, u32, rseq_len,
int, flags, u32, sig)
{
int ret;
+ u32 tls_flags;
if (flags & RSEQ_FLAG_UNREGISTER) {
if (flags & ~RSEQ_FLAG_UNREGISTER)
@@ -315,7 +322,7 @@ SYSCALL_DEFINE4(rseq, struct rseq __user *, rseq, u32, rseq_len,
/* Unregister rseq for current thread. */
if (current->rseq != rseq || !current->rseq)
return -EINVAL;
- if (rseq_len != sizeof(*rseq))
+ if (rseq_len != RSEQ_LEN_EXPECTED)
return -EINVAL;
if (current->rseq_sig != sig)
return -EPERM;
@@ -323,6 +330,7 @@ SYSCALL_DEFINE4(rseq, struct rseq __user *, rseq, u32, rseq_len,
if (ret)
return ret;
current->rseq = NULL;
+ current->rseq_size = 0;
current->rseq_sig = 0;
return 0;
}
@@ -336,7 +344,7 @@ SYSCALL_DEFINE4(rseq, struct rseq __user *, rseq, u32, rseq_len,
* the provided address differs from the prior
* one.
*/
- if (current->rseq != rseq || rseq_len != sizeof(*rseq))
+ if (current->rseq != rseq || rseq_len != RSEQ_LEN_EXPECTED)
return -EINVAL;
if (current->rseq_sig != sig)
return -EPERM;
@@ -349,10 +357,32 @@ SYSCALL_DEFINE4(rseq, struct rseq __user *, rseq, u32, rseq_len,
* ensure the provided rseq is properly aligned and valid.
*/
if (!IS_ALIGNED((unsigned long)rseq, __alignof__(*rseq)) ||
- rseq_len != sizeof(*rseq))
+ rseq_len != RSEQ_LEN_EXPECTED)
return -EINVAL;
- if (!access_ok(rseq, rseq_len))
+ if (!access_ok(rseq, RSEQ_LEN_EXPECTED))
return -EFAULT;
+
+ /* Handle extensible struct rseq ABI. */
+ ret = get_user(tls_flags, &rseq->flags);
+ if (ret)
+ return ret;
+ if (tls_flags & RSEQ_TLS_FLAG_SIZE) {
+ u16 user_size, kernel_size;
+
+ ret = get_user(user_size, &rseq->user_size);
+ if (ret)
+ return ret;
+ if (user_size < offsetof(struct rseq, kernel_size) + sizeof(u16))
+ return -EINVAL;
+ kernel_size = min_t(u16, user_size, offsetof(struct rseq, end));
+ ret = put_user(kernel_size, &rseq->kernel_size);
+ if (ret)
+ return ret;
+ current->rseq_size = kernel_size;
+ } else {
+ current->rseq_size = offsetof(struct rseq, user_size);
+ }
+
current->rseq = rseq;
current->rseq_sig = sig;
/*
--
2.17.1