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

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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>
This commit is contained in:
James Morse 2020-05-19 19:21:08 +01:00 committed by Will Deacon
parent 82b2077afc
commit 472de63b0b
1 changed files with 8 additions and 0 deletions
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8
drivers/firmware/arm_sdei.c
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@ -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);