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authorAndy Lutomirski <luto@amacapital.net>2014-12-04 16:48:17 -0800
committerIngo Molnar <mingo@kernel.org>2014-12-14 08:50:31 +0100
commit0e58af4e1d2166e9e33375a0f121e4867010d4f8 (patch)
treec0c467ac8bb54409c38026f0c925f4a57a8bdc0f
parent41bdc78544b8a93a9c6814b8bbbfef966272abbe (diff)
x86/tls: Disallow unusual TLS segments
Users have no business installing custom code segments into the GDT, and segments that are not present but are otherwise valid are a historical source of interesting attacks. For completeness, block attempts to set the L bit. (Prior to this patch, the L bit would have been silently dropped.) This is an ABI break. I've checked glibc, musl, and Wine, and none of them look like they'll have any trouble. Note to stable maintainers: this is a hardening patch that fixes no known bugs. Given the possibility of ABI issues, this probably shouldn't be backported quickly. Signed-off-by: Andy Lutomirski <luto@amacapital.net> Acked-by: H. Peter Anvin <hpa@zytor.com> Cc: stable@vger.kernel.org # optional Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: security@kernel.org <security@kernel.org> Cc: Willy Tarreau <w@1wt.eu> Signed-off-by: Ingo Molnar <mingo@kernel.org>
-rw-r--r--arch/x86/kernel/tls.c22
1 files changed, 22 insertions, 0 deletions
diff --git a/arch/x86/kernel/tls.c b/arch/x86/kernel/tls.c
index e7650bd71109..3e551eee87b9 100644
--- a/arch/x86/kernel/tls.c
+++ b/arch/x86/kernel/tls.c
@@ -39,6 +39,28 @@ static bool tls_desc_okay(const struct user_desc *info)
if (!info->seg_32bit)
return false;
+ /* Only allow data segments in the TLS array. */
+ if (info->contents > 1)
+ return false;
+
+ /*
+ * Non-present segments with DPL 3 present an interesting attack
+ * surface. The kernel should handle such segments correctly,
+ * but TLS is very difficult to protect in a sandbox, so prevent
+ * such segments from being created.
+ *
+ * If userspace needs to remove a TLS entry, it can still delete
+ * it outright.
+ */
+ if (info->seg_not_present)
+ return false;
+
+#ifdef CONFIG_X86_64
+ /* The L bit makes no sense for data. */
+ if (info->lm)
+ return false;
+#endif
+
return true;
}