CVE-2014-9090 (published 2014-11-30T01:59Z)

The do_double_fault function in arch/x86/kernel/traps.c in the Linux kernel through 3.17.4 does not properly handle faults associated with the Stack Segment (SS) segment register, which allows local users to cause a denial of service (panic) via a modify_ldt system call, as demonstrated by sigreturn_32 in the linux-clock-tests test suite.

Severity: MEDIUM
CVSS v2 base score: 4.9 (AV:L/AC:L/Au:N/C:N/I:N/A:C)

Weakness information:
cwe_id: CWE-17
cwe_name: DEPRECATED: Code
description: This entry has been deprecated.  It was originally used for organizing the Development View (CWE-699) and some other views, but it introduced unnecessary complexity and depth to the resulting tree.

Fix information:
repo_url: https://github.com/torvalds/linux
commit_details: repo_url: https://github.com/torvalds/linux
author: Andy Lutomirski
author_date: 2014-11-22 18:00:32-08:00
msg: x86_64, traps: Stop using IST for #SS

On a 32-bit kernel, this has no effect, since there are no IST stacks.

On a 64-bit kernel, #SS can only happen in user code, on a failed iret
to user space, a canonical violation on access via RSP or RBP, or a
genuine stack segment violation in 32-bit kernel code.  The first two
cases don't need IST, and the latter two cases are unlikely fatal bugs,
and promoting them to double faults would be fine.

This fixes a bug in which the espfix64 code mishandles a stack segment
violation.

This saves 4k of memory per CPU and a tiny bit of code.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
repository: repo_url: https://github.com/torvalds/linux
repo_name: torvalds/linux
description: Linux kernel source tree
repo_language: C
owner: torvalds
file_changes: filename: page_32_types.h
change_type: ModificationType.MODIFY
diff: @@ -20,7 +20,6 @@
 #define THREAD_SIZE_ORDER	1
 #define THREAD_SIZE		(PAGE_SIZE << THREAD_SIZE_ORDER)
 
-#define STACKFAULT_STACK 0
 #define DOUBLEFAULT_STACK 1
 #define NMI_STACK 0
 #define DEBUG_STACK 0

programming_language: C
filename: page_64_types.h
change_type: ModificationType.MODIFY
diff: @@ -14,12 +14,11 @@
 #define IRQ_STACK_ORDER 2
 #define IRQ_STACK_SIZE (PAGE_SIZE << IRQ_STACK_ORDER)
 
-#define STACKFAULT_STACK 1
-#define DOUBLEFAULT_STACK 2
-#define NMI_STACK 3
-#define DEBUG_STACK 4
-#define MCE_STACK 5
-#define N_EXCEPTION_STACKS 5  /* hw limit: 7 */
+#define DOUBLEFAULT_STACK 1
+#define NMI_STACK 2
+#define DEBUG_STACK 3
+#define MCE_STACK 4
+#define N_EXCEPTION_STACKS 4  /* hw limit: 7 */
 
 #define PUD_PAGE_SIZE		(_AC(1, UL) << PUD_SHIFT)
 #define PUD_PAGE_MASK		(~(PUD_PAGE_SIZE-1))

programming_language: Shell
filename: traps.h
change_type: ModificationType.MODIFY
diff: @@ -39,6 +39,7 @@ asmlinkage void simd_coprocessor_error(void);
 
 #ifdef CONFIG_TRACING
 asmlinkage void trace_page_fault(void);
+#define trace_stack_segment stack_segment
 #define trace_divide_error divide_error
 #define trace_bounds bounds
 #define trace_invalid_op invalid_op

programming_language: C
filename: dumpstack_64.c
change_type: ModificationType.MODIFY
diff: @@ -24,7 +24,6 @@ static char x86_stack_ids[][8] = {
 		[ DEBUG_STACK-1			]	= "#DB",
 		[ NMI_STACK-1			]	= "NMI",
 		[ DOUBLEFAULT_STACK-1		]	= "#DF",
-		[ STACKFAULT_STACK-1		]	= "#SS",
 		[ MCE_STACK-1			]	= "#MC",
 #if DEBUG_STKSZ > EXCEPTION_STKSZ
 		[ N_EXCEPTION_STACKS ...

programming_language: C
filename: entry_64.S
change_type: ModificationType.MODIFY
diff: @@ -1259,7 +1259,7 @@ apicinterrupt3 HYPERVISOR_CALLBACK_VECTOR \
 
 idtentry debug do_debug has_error_code=0 paranoid=1 shift_ist=DEBUG_STACK
 idtentry int3 do_int3 has_error_code=0 paranoid=1 shift_ist=DEBUG_STACK
-idtentry stack_segment do_stack_segment has_error_code=1 paranoid=1
+idtentry stack_segment do_stack_segment has_error_code=1
 #ifdef CONFIG_XEN
 idtentry xen_debug do_debug has_error_code=0
 idtentry xen_int3 do_int3 has_error_code=0

programming_language: C
filename: traps.c
change_type: ModificationType.MODIFY
diff: @@ -233,27 +233,11 @@ DO_ERROR(X86_TRAP_UD,     SIGILL,  "invalid opcode",		invalid_op)
 DO_ERROR(X86_TRAP_OLD_MF, SIGFPE,  "coprocessor segment overrun",coprocessor_segment_overrun)
 DO_ERROR(X86_TRAP_TS,     SIGSEGV, "invalid TSS",		invalid_TSS)
 DO_ERROR(X86_TRAP_NP,     SIGBUS,  "segment not present",	segment_not_present)
-#ifdef CONFIG_X86_32
 DO_ERROR(X86_TRAP_SS,     SIGBUS,  "stack segment",		stack_segment)
-#endif
 DO_ERROR(X86_TRAP_AC,     SIGBUS,  "alignment check",		alignment_check)
 
 #ifdef CONFIG_X86_64
 /* Runs on IST stack */
-dotraplinkage void do_stack_segment(struct pt_regs *regs, long error_code)
-{
-	enum ctx_state prev_state;
-
-	prev_state = exception_enter();
-	if (notify_die(DIE_TRAP, "stack segment", regs, error_code,
-		       X86_TRAP_SS, SIGBUS) != NOTIFY_STOP) {
-		preempt_conditional_sti(regs);
-		do_trap(X86_TRAP_SS, SIGBUS, "stack segment", regs, error_code, NULL);
-		preempt_conditional_cli(regs);
-	}
-	exception_exit(prev_state);
-}
-
 dotraplinkage void do_double_fault(struct pt_regs *regs, long error_code)
 {
 	static const char str[] = "double fault";
@@ -802,7 +786,7 @@ void __init trap_init(void)
 	set_intr_gate(X86_TRAP_OLD_MF, coprocessor_segment_overrun);
 	set_intr_gate(X86_TRAP_TS, invalid_TSS);
 	set_intr_gate(X86_TRAP_NP, segment_not_present);
-	set_intr_gate_ist(X86_TRAP_SS, &stack_segment, STACKFAULT_STACK);
+	set_intr_gate(X86_TRAP_SS, stack_segment);
 	set_intr_gate(X86_TRAP_GP, general_protection);
 	set_intr_gate(X86_TRAP_SPURIOUS, spurious_interrupt_bug);
 	set_intr_gate(X86_TRAP_MF, coprocessor_error);

programming_language: C