This is a preparatory patch for enabling checkpoint/restart of tasks utilizing syscall user dispatch via ptrace. To support checkpoint/restart, ptrace must be able to set the selector of the tracee. The selector is a user pointer that may be subject to memory tagging extensions on some architectures (namely ARM MTE). access_ok will clear memory tags for tagged addresses on tasks where memory tagging is enabled. However, to allow ptrace to set a task's selector when tracer and tracee are not both tagged or untagged, the selector address must be untagged when calling access_ok. Since access_ok utilizes current to determine whether or not to untag an address, an untagged tracer will always fail to restore a tagged address in a tagged tracee. This patch will resolve this issue. The result of this is that a tagged tracer may be capable of setting an invalid address, which will cause the tracee to SIGSEGV on next syscall. This is equivalent to the tracee setting a bad selector address (such as selector=0x1). This is preferable to the alternative of creating a task_access_ok variant, and is consistent with other operations which change tracee pointers via ptrace. For more information, see: https://lore.kernel.org/all/ZCWXE04nLZ4pXEtM@xxxxxxx/ Signed-off-by: Gregory Price <gregory.price@xxxxxxxxxxxx> Suggested-by: Catalin Marinas <catalin.marinas@xxxxxxx> --- kernel/entry/syscall_user_dispatch.c | 13 ++++++++++++- 1 file changed, 12 insertions(+), 1 deletion(-) diff --git a/kernel/entry/syscall_user_dispatch.c b/kernel/entry/syscall_user_dispatch.c index 22396b234854..16086226b41c 100644 --- a/kernel/entry/syscall_user_dispatch.c +++ b/kernel/entry/syscall_user_dispatch.c @@ -87,7 +87,18 @@ static int task_set_syscall_user_dispatch(struct task_struct *task, unsigned lon if (offset && offset + len <= offset) return -EINVAL; - if (selector && !access_ok(selector, sizeof(*selector))) + /* + * access_ok will clear memory tags for tagged addresses on tasks where + * memory tagging is enabled. To enable a tracer to set a tracee's + * selector not in the same tagging state, the selector address must be + * untagged for access_ok, otherwise an untagged tracer will always fail + * to set a tagged tracee's selector. + * + * The result of this is that a tagged tracer may be capable of setting + * an invalid address, and the tracee will SIGSEGV on the next syscall. + * This is equivalent to a task setting a bad selector (selector=0x1). + */ + if (selector && !access_ok(untagged_addr(selector), sizeof(*selector))) return -EFAULT; break; -- 2.39.1
