From: James Morse <james.morse@arm.com>
Date: Tue, 19 May 2020 19:21:08 +0100
Subject: firmware: arm_sdei: Document the motivation behind these set_fs()
 calls
Git-commit: 472de63b0b8383565e103f809f5df37d1c4390ab
Patch-mainline: v5.8-rc1
References: jsc#SLE-16610

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>
Signed-off-by: Matthias Brugger <mbrugger@suse.com>
---
 drivers/firmware/arm_sdei.c |    8 ++++++++
 1 file changed, 8 insertions(+)

--- a/drivers/firmware/arm_sdei.c
+++ b/drivers/firmware/arm_sdei.c
@@ -1147,6 +1147,14 @@ int sdei_event_handler(struct pt_regs *r
 	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);