linux-kernel - [PATCH 3.16 165/294] x86/ldt: Fix off by one in get_segment

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Date:   Mon, 06 Nov 2017 23:03:02 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, kernel-janitors@...r.kernel.org,
        "Arnaldo Carvalho de Melo" <acme@...nel.org>,
        "Ingo Molnar" <mingo@...nel.org>,
        "Andy Lutomirski" <luto@...nel.org>,
        "Alexander Shishkin" <alexander.shishkin@...ux.intel.com>,
        "Peter Zijlstra" <peterz@...radead.org>,
        "Dan Carpenter" <dan.carpenter@...cle.com>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        "Linus Torvalds" <torvalds@...ux-foundation.org>
Subject: [PATCH 3.16 165/294] x86/ldt: Fix off by one in get_segment_base()

3.16.50-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Dan Carpenter <dan.carpenter@...cle.com>

commit eaa2f87c6b840b83827c40db6eb8481689570259 upstream.

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>
[bwh: Backported to 3.16: adjust filename, context]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 arch/x86/kernel/cpu/perf_event.c | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

--- a/arch/x86/kernel/cpu/perf_event.c
+++ b/arch/x86/kernel/cpu/perf_event.c
@@ -2009,17 +2009,14 @@ static unsigned long get_segment_base(un
 	if ((segment & SEGMENT_TI_MASK) == SEGMENT_LDT) {
 		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->size)
+		if (!ldt || idx >= ldt->size)
 			return 0;
 
 		desc = &ldt->entries[idx];
 	} else {
-		if (idx > GDT_ENTRIES)
+		if (idx >= GDT_ENTRIES)
 			return 0;
 
 		desc = __this_cpu_ptr(&gdt_page.gdt[0]) + idx;