firmware: arm_sdei: Document the motivation behind these set_fs() calls

The SDEI handler save/restores the addr_limit using set_fs(). It isn't
very clear why. The reason is to mirror the arch code's entry assembly.
The arch code does this because perf may access user-space, and
inheriting the addr_limit may be a problem.

Add a comment explaining why this is here.

Suggested-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: James Morse <james.morse@arm.com>
Link: https://bugs.chromium.org/p/project-zero/issues/detail?id=822
Link: https://lore.kernel.org/r/20200519182108.13693-4-james.morse@arm.com
Signed-off-by: Will Deacon <will@kernel.org>

diff --git a/drivers/firmware/arm_sdei.c b/drivers/firmware/arm_sdei.c
index b12b99a..e7e36aa 100644 (file)
--- a/drivers/firmware/arm_sdei.c
+++ b/drivers/firmware/arm_sdei.c
@@ -1128,6 +1128,14 @@ int sdei_event_handler(struct pt_regs *regs,
        mm_segment_t orig_addr_limit;
        u32 event_num = arg->event_num;
 
+       /*
+        * Save restore 'fs'.
+        * The architecture's entry code save/restores 'fs' when taking an
+        * exception from the kernel. This ensures addr_limit isn't inherited
+        * if you interrupted something that allowed the uaccess routines to
+        * access kernel memory.
+        * Do the same here because this doesn't come via the same entry code.
+       */
        orig_addr_limit = get_fs();
        set_fs(USER_DS);