[PATCH v16 2/4] syscall user dispatch: untag selector addresses before access_ok

[Gregory Price]

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>
Acked-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Suggested-by: Catalin Marinas <catalin.marinas@xxxxxxx>
Reviewed-by: Catalin Marinas <catalin.marinas@xxxxxxx>
---
 kernel/entry/syscall_user_dispatch.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/kernel/entry/syscall_user_dispatch.c b/kernel/entry/syscall_user_dispatch.c
index 22396b234854..424f24350f8b 100644
--- a/kernel/entry/syscall_user_dispatch.c
+++ b/kernel/entry/syscall_user_dispatch.c
@@ -87,7 +87,14 @@ 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.
+		 */
+		if (selector && !access_ok(untagged_addr(selector), sizeof(*selector)))
 			return -EFAULT;
 
 		break;
-- 
2.39.1