[PATCH 2/3] x86/dumpstack: Inline copy_from_user_nmi()

Kees Cook
2022-09-16T14:01:54Z

The check_object_size() helper under CONFIG_HARDENED_USERCOPY is
designed to skip any checks where the length is known at compile time as
a reasonable heuristic to avoid "likely known-good" cases. However, it can
only do this when the copy_*_user() helpers are, themselves, inline too.

Using find_vmap_area() requires taking a spinlock. The check_object_size()
helper can call find_vmap_area() when the destination is in vmap memory.
If show_regs() is called in interrupt context, it will attempt a call to
copy_from_user_nmi(), which may call check_object_size() and then
find_vmap_area(). If something in normal context happens to be in the
middle of calling find_vmap_area() (with the spinlock held), the interrupt
handler will hang forever.

The copy_from_user_nmi() call is actually being called with a fixed-size
length, so check_object_size() should never have been called in the
first place. In order for check_object_size() to see that the length is
a fixed size, inline copy_from_user_nmi(), as already done with all the
other uaccess helpers.

Reported-by: Yu Zhao <yuzhao@google.com>
Link: https://lore.kernel.org/all/CAOUHufaPshtKrTWOz7T7QFYUNVGFm0JBjvM700Nhf9qEL9b3EQ@mail.gmail.com
Reported-by: dev@der-flo.net
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: x86@kernel.org
Fixes: 0aef499f3172 ("mm/usercopy: Detect vmalloc overruns")
Cc: stable@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
---
arch/x86/include/asm/uaccess.h | 2 --
arch/x86/kernel/dumpstack.c | 4 +--
arch/x86/lib/Makefile | 2 +-
arch/x86/lib/usercopy.c | 50 ----------------------------------
include/linux/uaccess.h | 41 ++++++++++++++++++++++++++++
5 files changed, 44 insertions(+), 55 deletions(-)
delete mode 100644 arch/x86/lib/usercopy.c

diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index e9390eea861b..f47c0c752e7a 100644
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -498,8 +498,6 @@ struct __large_struct { unsigned long buf[100]; };
: : ltype(x), "m" (__m(addr)) \
: : label)

-extern unsigned long
-copy_from_user_nmi(void *to, const void __user *from, unsigned long n);
extern __must_check long
strncpy_from_user(char *dst, const char __user *src, long count);

diff --git a/arch/x86/kernel/dumpstack.c b/arch/x86/kernel/dumpstack.c
index afae4dd77495..b59d59ef10d2 100644
--- a/arch/x86/kernel/dumpstack.c
+++ b/arch/x86/kernel/dumpstack.c
@@ -72,8 +72,8 @@ static void printk_stack_address(unsigned long address, int reliable,
printk("%s %s%pBb\n", log_lvl, reliable ? "" : "? ", (void *)address);
}

-static int copy_code(struct pt_regs *regs, u8 *buf, unsigned long src,
- unsigned int nbytes)
+static __always_inline int
+copy_code(struct pt_regs *regs, u8 *buf, unsigned long src, unsigned int nbytes)
{
if (!user_mode(regs))
return copy_from_kernel_nofault(buf, (u8 *)src, nbytes);
diff --git a/arch/x86/lib/Makefile b/arch/x86/lib/Makefile
index f76747862bd2..aeb5cd634e27 100644
--- a/arch/x86/lib/Makefile
+++ b/arch/x86/lib/Makefile
@@ -42,7 +42,7 @@ clean-files := inat-tables.c
obj-$(CONFIG_SMP) += msr-smp.o cache-smp.o

lib-y := delay.o misc.o cmdline.o cpu.o
-lib-y += usercopy_$(BITS).o usercopy.o getuser.o putuser.o
+lib-y += usercopy_$(BITS).o getuser.o putuser.o
lib-y += memcpy_$(BITS).o
lib-y += pc-conf-reg.o
lib-$(CONFIG_ARCH_HAS_COPY_MC) += copy_mc.o copy_mc_64.o
diff --git a/arch/x86/lib/usercopy.c b/arch/x86/lib/usercopy.c
deleted file mode 100644
index 959489f2f814..000000000000
--- a/arch/x86/lib/usercopy.c
+++ /dev/null
@@ -1,50 +0,0 @@
-/*
- * User address space access functions.
- *
- * For licencing details see kernel-base/COPYING
- */
-
-#include <linux/uaccess.h>
-#include <linux/export.h>
-
-/**
- * copy_from_user_nmi - NMI safe copy from user
- * @to: Pointer to the destination buffer
- * @from: Pointer to a user space address of the current task
- * @n: Number of bytes to copy
- *
- * Returns: The number of not copied bytes. 0 is success, i.e. all bytes copied
- *
- * Contrary to other copy_from_user() variants this function can be called
- * from NMI context. Despite the name it is not restricted to be called
- * from NMI context. It is safe to be called from any other context as
- * well. It disables pagefaults across the copy which means a fault will
- * abort the copy.
- *
- * For NMI context invocations this relies on the nested NMI work to allow
- * atomic faults from the NMI path; the nested NMI paths are careful to
- * preserve CR2.
- */
-unsigned long
-copy_from_user_nmi(void *to, const void __user *from, unsigned long n)
-{
- unsigned long ret;
-
- if (!__access_ok(from, n))
- return n;
-
- if (!nmi_uaccess_okay())
- return n;
-
- /*
- * Even though this function is typically called from NMI/IRQ context
- * disable pagefaults so that its behaviour is consistent even when
- * called from other contexts.
- */
- pagefault_disable();
- ret = __copy_from_user_inatomic(to, from, n);
- pagefault_enable();
-
- return ret;
-}
-EXPORT_SYMBOL_GPL(copy_from_user_nmi);
diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
index 065e121d2a86..fee141ed8f95 100644
--- a/include/linux/uaccess.h
+++ b/include/linux/uaccess.h
@@ -273,6 +273,47 @@ __copy_from_user_inatomic_nocache(void *to, const void __user *from,

#endif /* ARCH_HAS_NOCACHE_UACCESS */

+/**
+ * copy_from_user_nmi - NMI safe copy from user
+ * @to: Pointer to the destination buffer
+ * @from: Pointer to a user space address of the current task
+ * @n: Number of bytes to copy
+ *
+ * Returns: The number of not copied bytes. 0 is success, i.e. all bytes copied
+ *
+ * Contrary to other copy_from_user() variants this function can be called
+ * from NMI context. Despite the name it is not restricted to be called
+ * from NMI context. It is safe to be called from any other context as
+ * well. It disables pagefaults across the copy which means a fault will
+ * abort the copy.
+ *
+ * For NMI context invocations this relies on the nested NMI work to allow
+ * atomic faults from the NMI path; the nested NMI paths are careful to
+ * preserve CR2.
+ */
+static __always_inline unsigned long
+copy_from_user_nmi(void *to, const void __user *from, const unsigned long n)
+{
+ unsigned long ret;
+
+ if (!__access_ok(from, n))
+ return n;
+
+ if (!nmi_uaccess_okay())
+ return n;
+
+ /*
+ * Even though this function is typically called from NMI/IRQ context
+ * disable pagefaults so that its behaviour is consistent even when
+ * called from other contexts.
+ */
+ pagefault_disable();
+ ret = __copy_from_user_inatomic(to, from, n);
+ pagefault_enable();
+
+ return ret;
+}
+
extern __must_check int check_zeroed_user(const void __user *from, size_t size);

/**
--
2.34.1