linux-kernel - [tip:x86/urgent] x86/ldt: Fix off by one in get_segment

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Message-ID: <tip-eaa2f87c6b840b83827c40db6eb8481689570259@git.kernel.org>
Date:   Tue, 29 Aug 2017 04:11:57 -0700
From:   tip-bot for Dan Carpenter <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     peterz@...radead.org, linux-kernel@...r.kernel.org,
        alexander.shishkin@...ux.intel.com, mingo@...nel.org,
        hpa@...or.com, acme@...nel.org, tglx@...utronix.de,
        dan.carpenter@...cle.com, torvalds@...ux-foundation.org,
        luto@...nel.org
Subject: [tip:x86/urgent] x86/ldt: Fix off by one in get_segment_base()

Commit-ID:  eaa2f87c6b840b83827c40db6eb8481689570259
Gitweb:     http://git.kernel.org/tip/eaa2f87c6b840b83827c40db6eb8481689570259
Author:     Dan Carpenter <dan.carpenter@...cle.com>
AuthorDate: Fri, 18 Aug 2017 13:30:30 +0300
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Tue, 29 Aug 2017 11:55:15 +0200

x86/ldt: Fix off by one in get_segment_base()

ldt->entries[] is allocated in alloc_ldt_struct().  It has
ldt->nr_entries elements and ldt->nr_entries is capped at LDT_ENTRIES.
So if "idx" is == ldt->nr_entries then we're reading beyond the end of
the buffer.  It seems duplicative to have two limit checks when one
would work just as well so I removed the check against LDT_ENTRIES.

The gdt_page.gdt[] array has GDT_ENTRIES entries.

Signed-off-by: Dan Carpenter <dan.carpenter@...cle.com>
Acked-by: Andy Lutomirski <luto@...nel.org>
Cc: Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc: Arnaldo Carvalho de Melo <acme@...nel.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: kernel-janitors@...r.kernel.org
Fixes: d07bdfd322d3 ("perf/x86: Fix USER/KERNEL tagging of samples properly")
Link: http://lkml.kernel.org/r/20170818102516.gqwm4xdvvuvjw5ho@mwanda
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 arch/x86/events/core.c | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
index af12e29..9390501 100644
--- a/arch/x86/events/core.c
+++ b/arch/x86/events/core.c
@@ -2335,12 +2335,9 @@ static unsigned long get_segment_base(unsigned int segment)
 #ifdef CONFIG_MODIFY_LDT_SYSCALL
 		struct ldt_struct *ldt;
 
-		if (idx > LDT_ENTRIES)
-			return 0;
-
 		/* IRQs are off, so this synchronizes with smp_store_release */
 		ldt = lockless_dereference(current->active_mm->context.ldt);
-		if (!ldt || idx > ldt->nr_entries)
+		if (!ldt || idx >= ldt->nr_entries)
 			return 0;
 
 		desc = &ldt->entries[idx];
@@ -2348,7 +2345,7 @@ static unsigned long get_segment_base(unsigned int segment)
 		return 0;
 #endif
 	} else {
-		if (idx > GDT_ENTRIES)
+		if (idx >= GDT_ENTRIES)
 			return 0;
 
 		desc = raw_cpu_ptr(gdt_page.gdt) + idx;