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Subject[tip: x86/cleanups] x86/uaccess: Document copy_from_user_nmi()
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@linutronix.de>
AuthorDate: Tue, 17 Nov 2020 21:23:35 +01:00
Committer: Borislav Petkov <bp@suse.de>
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@alien8.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20201117202753.806376613@linutronix.de
---
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);
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