[PATCH v15 2/4] syscall user dispatch: untag selector addresses before access_ok

Gregory Price
2023-03-30T21:22:01Z

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@arm.com/

Signed-off-by: Gregory Price <gregory.price@memverge.com>
Suggested-by: Catalin Marinas <catalin.marinas@arm.com>
---
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