[tip: x86/cleanups] x86/uaccess: Document copy_from_user_nmi()

[tip-bot2 for Thomas Gleixner]

The following commit has been merged into the x86/cleanups branch of tip:

Commit-ID:     907f8eb8e0eb2b3312b292e67dc4dbc493424747
Gitweb:        https://git.kernel.org/tip/907f8eb8e0eb2b3312b292e67dc4dbc493424747
Author:        Thomas Gleixner <tglx@xxxxxxxxxxxxx>
AuthorDate:    Tue, 17 Nov 2020 21:23:35 +01:00
Committer:     Borislav Petkov <bp@xxxxxxx>
CommitterDate: Wed, 18 Nov 2020 13:19:01 +01:00

x86/uaccess: Document copy_from_user_nmi()

Document the functionality of copy_from_user_nmi() to avoid further
confusion. Fix the typo in the existing comment while at it.

Requested-by: Borislav Petkov <bp@xxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Link: https://lkml.kernel.org/r/20201117202753.806376613@xxxxxxxxxxxxx
---
 arch/x86/lib/usercopy.c | 22 ++++++++++++++++++----
 1 file changed, 18 insertions(+), 4 deletions(-)

diff --git a/arch/x86/lib/usercopy.c b/arch/x86/lib/usercopy.c
index 3f435d7..c3e8a62 100644
--- a/arch/x86/lib/usercopy.c
+++ b/arch/x86/lib/usercopy.c
@@ -9,9 +9,23 @@
 
 #include <asm/tlbflush.h>
 
-/*
- * We rely on the nested NMI work to allow atomic faults from the NMI path; the
- * nested NMI paths are careful to preserve CR2.
+/**
+ * copy_from_user_nmi - NMI safe copy from user
+ * @to:		Pointer to the destination buffer
+ * @from:	Pointer to a user space address of the current task
+ * @n:		Number of bytes to copy
+ *
+ * Returns: The number of not copied bytes. 0 is success, i.e. all bytes copied
+ *
+ * Contrary to other copy_from_user() variants this function can be called
+ * from NMI context. Despite the name it is not restricted to be called
+ * from NMI context. It is safe to be called from any other context as
+ * well. It disables pagefaults across the copy which means a fault will
+ * abort the copy.
+ *
+ * For NMI context invocations this relies on the nested NMI work to allow
+ * atomic faults from the NMI path; the nested NMI paths are careful to
+ * preserve CR2.
  */
 unsigned long
 copy_from_user_nmi(void *to, const void __user *from, unsigned long n)
@@ -27,7 +41,7 @@ copy_from_user_nmi(void *to, const void __user *from, unsigned long n)
 	/*
 	 * Even though this function is typically called from NMI/IRQ context
 	 * disable pagefaults so that its behaviour is consistent even when
-	 * called form other contexts.
+	 * called from other contexts.
 	 */
 	pagefault_disable();
 	ret = __copy_from_user_inatomic(to, from, n);