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Date: Tue, 21 Nov 2017 16:54:32 -0800
From: Matthew Garrett <mjg59@...gle.com>
To: keescook@...omium.org
Cc: kernel-hardening@...ts.openwall.com, Matthew Garrett <mjg59@...gle.com>
Subject: [PATCH] usercopy: Allow runtime disabling of usercopy fallback

Distributions are likely to build with
CONFIG_HARDENED_USERCOPY_FALLBACK, but individual sites may wish to
disable that functionality. Add a sysctl that allows the fallback to be
disabled and then forbids any attempt to re-enable it.

Signed-off-by: Matthew Garrett <mjg59@...gle.com>
---
 Documentation/sysctl/vm.txt |  9 +++++++++
 include/linux/mm.h          |  7 +++++++
 kernel/sysctl.c             | 11 +++++++++++
 mm/memory.c                 | 17 +++++++++++++++++
 mm/slab.c                   |  4 +---
 mm/slub.c                   |  4 +---
 6 files changed, 46 insertions(+), 6 deletions(-)

diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt
index 9baf66a9ef4e..b494b292fd93 100644
--- a/Documentation/sysctl/vm.txt
+++ b/Documentation/sysctl/vm.txt
@@ -60,6 +60,7 @@ Currently, these files are in /proc/sys/vm:
 - stat_refresh
 - swappiness
 - user_reserve_kbytes
+- usercopy_fallback
 - vfs_cache_pressure
 - watermark_scale_factor
 - zone_reclaim_mode
@@ -822,6 +823,14 @@ Changing this takes effect whenever an application requests memory.
 
 ==============================================================
 
+usercopy_fallback
+
+If 1, attempts to copy data from userland will generate a kernel warning
+if the target has not been whitelisted, but will still be permitted. If 0,
+this will result in a kernel BUG(). This may be set from 1 to 0 at runtime,
+but may not be modified if 0.
+
+==============================================================
 vfs_cache_pressure
 ------------------
 
diff --git a/include/linux/mm.h b/include/linux/mm.h
index f8c10d336e42..511237addbf9 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2456,6 +2456,8 @@ int drop_caches_sysctl_handler(struct ctl_table *, int,
 void drop_slab(void);
 void drop_slab_node(int nid);
 
+extern bool usercopy_fallback;
+
 #ifndef CONFIG_MMU
 #define randomize_va_space 0
 #else
@@ -2602,5 +2604,10 @@ void __init setup_nr_node_ids(void);
 static inline void setup_nr_node_ids(void) {}
 #endif
 
+#ifdef CONFIG_HARDENED_USERCOPY
+int usercopy_fallback_handler(struct ctl_table *table, int write,
+			    void __user *buffer, size_t *length, loff_t *ppos);
+#endif
+
 #endif /* __KERNEL__ */
 #endif /* _LINUX_MM_H */
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 423554ad3610..ba3366335b40 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -1663,6 +1663,17 @@ static struct ctl_table vm_table[] = {
 		.extra1		= (void *)&mmap_rnd_compat_bits_min,
 		.extra2		= (void *)&mmap_rnd_compat_bits_max,
 	},
+#endif
+#ifdef CONFIG_HARDENED_USERCOPY
+	{
+		.procname	= "usercopy_fallback",
+		.data		= &usercopy_fallback,
+		.maxlen		= sizeof(int),
+		.mode		= 0600,
+		.proc_handler	= usercopy_fallback_handler,
+		.extra1		= &zero,
+		.extra2		= &one,
+	},
 #endif
 	{ }
 };
diff --git a/mm/memory.c b/mm/memory.c
index ec4e15494901..29ed21083a8d 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -104,6 +104,11 @@ EXPORT_SYMBOL(mem_map);
 void *high_memory;
 EXPORT_SYMBOL(high_memory);
 
+#ifdef CONFIG_HARDENED_USERCOPY
+/* Permit non-whitelisted usercopy targets */
+bool usercopy_fallback = IS_ENABLED(CONFIG_HARDENED_USERCOPY_FALLBACK);
+#endif
+
 /*
  * Randomize the address space (stacks, mmaps, brk, etc.).
  *
@@ -4668,3 +4673,15 @@ void ptlock_free(struct page *page)
 	kmem_cache_free(page_ptl_cachep, page->ptl);
 }
 #endif
+
+#ifdef CONFIG_HARDENED_USERCOPY
+int usercopy_fallback_handler(struct ctl_table *table, int write,
+			    void __user *buffer, size_t *length, loff_t *ppos)
+{
+	/* Don't allow re-enabling fallback if it's been disabled */
+	if (write && !usercopy_fallback)
+		return -EINVAL;
+
+	return proc_dointvec_minmax(table, write, buffer, length, ppos);
+}
+#endif
diff --git a/mm/slab.c b/mm/slab.c
index 23c53dc46cd6..6afcac1d8b97 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -4435,21 +4435,19 @@ const char *__check_heap_object(const void *ptr, unsigned long n,
 	if (offset < cachep->useroffset ||
 	    offset - cachep->useroffset > cachep->usersize ||
 	    n > cachep->useroffset - offset + cachep->usersize) {
-#ifdef CONFIG_HARDENED_USERCOPY_FALLBACK
 		/*
 		 * If no whitelist exists, and FALLBACK is set, produce
 		 * a warning instead of rejecting the copy. This is intended
 		 * to be a temporary method to find any missing usercopy
 		 * whitelists.
 		 */
-		if (cachep->usersize == 0 &&
+		if (usercopy_fallback && cachep->usersize == 0 &&
 		    offset <= cachep->object_size &&
 		    n <= cachep->object_size - offset) {
 			WARN_ONCE(1, "unexpected usercopy without slab whitelist from %s offset %lu size %lu",
 				  cachep->name, offset, n);
 			return NULL;
 		}
-#endif
 		return cachep->name;
 	}
 
diff --git a/mm/slub.c b/mm/slub.c
index 82e7f15162a8..e664c6e715e6 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -3867,7 +3867,6 @@ const char *__check_heap_object(const void *ptr, unsigned long n,
 	if (offset < s->useroffset ||
 	    offset - s->useroffset > s->usersize ||
 	    n > s->useroffset - offset + s->usersize) {
-#ifdef CONFIG_HARDENED_USERCOPY_FALLBACK
 		size_t object_size;
 
 		object_size = slab_ksize(s);
@@ -3878,13 +3877,12 @@ const char *__check_heap_object(const void *ptr, unsigned long n,
 		 * to be a temporary method to find any missing usercopy
 		 * whitelists.
 		 */
-		if (s->usersize == 0 &&
+		if (usercopy_fallback && s->usersize == 0 &&
 		    (offset <= object_size && n <= object_size - offset)) {
 			WARN_ONCE(1, "unexpected usercopy without slab whitelist from %s offset %lu size %lu",
 				  s->name, offset, n);
 			return NULL;
 		}
-#endif
 		return s->name;
 	}
 
-- 
2.15.0.448.gf294e3d99a-goog

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