arm64: uaccess: Fix omissions from usercopy whitelist

When the hardend usercopy support was added for arm64, it was
concluded that all cases of usercopy into and out of thread_struct
were statically sized and so didn't require explicit whitelisting
of the appropriate fields in thread_struct.

Testing with usercopy hardening enabled has revealed that this is
not the case for certain ptrace regset manipulation calls on arm64.
This occurs because the sizes of usercopies associated with the
regset API are dynamic by construction, and because arm64 does not
always stage such copies via the stack: indeed the regset API is
designed to avoid the need for that by adding some bounds checking.

This is currently believed to affect only the fpsimd and TLS
registers.

Because the whitelisted fields in thread_struct must be contiguous,
this patch groups them together in a nested struct.  It is also
necessary to be able to determine the location and size of that
struct, so rather than making the struct anonymous (which would
save on edits elsewhere) or adding an anonymous union containing
named and unnamed instances of the same struct (gross), this patch
gives the struct a name and makes the necessary edits to code that
references it (noisy but simple).

Care is needed to ensure that the new struct does not contain
padding (which the usercopy hardening would fail to protect).

For this reason, the presence of tp2_value is made unconditional,
since a padding field would be needed there in any case.  This pads
up to the 16-byte alignment required by struct user_fpsimd_state.

Acked-by: Kees Cook <keescook@chromium.org>
Reported-by: Mark Rutland <mark.rutland@arm.com>
Fixes: 9e8084d3f761 ("arm64: Implement thread_struct whitelist for hardened usercopy")
Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

1 files changed, 24 insertions, 23 deletions

diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
index b13a9b4dd13b..87a35364e750 100644
--- a/arch/arm64/kernel/fpsimd.c
+++ b/arch/arm64/kernel/fpsimd.c
@@ -224,7 +224,7 @@ static void sve_user_enable(void)
  *    sets TIF_SVE.
  *
  *    When stored, FPSIMD registers V0-V31 are encoded in
- *    task->fpsimd_state; bits [max : 128] for each of Z0-Z31 are
+ *    task->thread.uw.fpsimd_state; bits [max : 128] for each of Z0-Z31 are
  *    logically zero but not stored anywhere; P0-P15 and FFR are not
  *    stored and have unspecified values from userspace's point of
  *    view.  For hygiene purposes, the kernel zeroes them on next use,
@@ -233,9 +233,9 @@ static void sve_user_enable(void)
  *    task->thread.sve_state does not need to be non-NULL, valid or any
  *    particular size: it must not be dereferenced.
  *
- *  * FPSR and FPCR are always stored in task->fpsimd_state irrespctive of
- *    whether TIF_SVE is clear or set, since these are not vector length
- *    dependent.
+ *  * FPSR and FPCR are always stored in task->thread.uw.fpsimd_state
+ *    irrespective of whether TIF_SVE is clear or set, since these are
+ *    not vector length dependent.
  */
 
 /*
@@ -253,10 +253,10 @@ static void task_fpsimd_load(void)
 
 	if (system_supports_sve() && test_thread_flag(TIF_SVE))
 		sve_load_state(sve_pffr(current),
-			       &current->thread.fpsimd_state.fpsr,
+			       &current->thread.uw.fpsimd_state.fpsr,
 			       sve_vq_from_vl(current->thread.sve_vl) - 1);
 	else
-		fpsimd_load_state(&current->thread.fpsimd_state);
+		fpsimd_load_state(&current->thread.uw.fpsimd_state);
 
 	if (system_supports_sve()) {
 		/* Toggle SVE trapping for userspace if needed */
@@ -292,9 +292,9 @@ static void task_fpsimd_save(void)
 			}
 
 			sve_save_state(sve_pffr(current),
-				       &current->thread.fpsimd_state.fpsr);
+				       &current->thread.uw.fpsimd_state.fpsr);
 		} else
-			fpsimd_save_state(&current->thread.fpsimd_state);
+			fpsimd_save_state(&current->thread.uw.fpsimd_state);
 	}
 }
 
@@ -405,20 +405,21 @@ static int __init sve_sysctl_init(void) { return 0; }
 	(SVE_SIG_ZREG_OFFSET(vq, n) - SVE_SIG_REGS_OFFSET))
 
 /*
- * Transfer the FPSIMD state in task->thread.fpsimd_state to
+ * Transfer the FPSIMD state in task->thread.uw.fpsimd_state to
  * task->thread.sve_state.
  *
  * Task can be a non-runnable task, or current.  In the latter case,
  * softirqs (and preemption) must be disabled.
  * task->thread.sve_state must point to at least sve_state_size(task)
  * bytes of allocated kernel memory.
- * task->thread.fpsimd_state must be up to date before calling this function.
+ * task->thread.uw.fpsimd_state must be up to date before calling this
+ * function.
  */
 static void fpsimd_to_sve(struct task_struct *task)
 {
 	unsigned int vq;
 	void *sst = task->thread.sve_state;
-	struct user_fpsimd_state const *fst = &task->thread.fpsimd_state;
+	struct user_fpsimd_state const *fst = &task->thread.uw.fpsimd_state;
 	unsigned int i;
 
 	if (!system_supports_sve())
@@ -432,7 +433,7 @@ static void fpsimd_to_sve(struct task_struct *task)
 
 /*
  * Transfer the SVE state in task->thread.sve_state to
- * task->thread.fpsimd_state.
+ * task->thread.uw.fpsimd_state.
  *
  * Task can be a non-runnable task, or current.  In the latter case,
  * softirqs (and preemption) must be disabled.
@@ -444,7 +445,7 @@ static void sve_to_fpsimd(struct task_struct *task)
 {
 	unsigned int vq;
 	void const *sst = task->thread.sve_state;
-	struct user_fpsimd_state *fst = &task->thread.fpsimd_state;
+	struct user_fpsimd_state *fst = &task->thread.uw.fpsimd_state;
 	unsigned int i;
 
 	if (!system_supports_sve())
@@ -511,7 +512,7 @@ void fpsimd_sync_to_sve(struct task_struct *task)
 }
 
 /*
- * Ensure that task->thread.fpsimd_state is up to date with respect to
+ * Ensure that task->thread.uw.fpsimd_state is up to date with respect to
  * the user task, irrespective of whether SVE is in use or not.
  *
  * This should only be called by ptrace.  task must be non-runnable.
@@ -526,21 +527,21 @@ void sve_sync_to_fpsimd(struct task_struct *task)
 
 /*
  * Ensure that task->thread.sve_state is up to date with respect to
- * the task->thread.fpsimd_state.
+ * the task->thread.uw.fpsimd_state.
  *
  * This should only be called by ptrace to merge new FPSIMD register
  * values into a task for which SVE is currently active.
  * task must be non-runnable.
  * task->thread.sve_state must point to at least sve_state_size(task)
  * bytes of allocated kernel memory.
- * task->thread.fpsimd_state must already have been initialised with
+ * task->thread.uw.fpsimd_state must already have been initialised with
  * the new FPSIMD register values to be merged in.
  */
 void sve_sync_from_fpsimd_zeropad(struct task_struct *task)
 {
 	unsigned int vq;
 	void *sst = task->thread.sve_state;
-	struct user_fpsimd_state const *fst = &task->thread.fpsimd_state;
+	struct user_fpsimd_state const *fst = &task->thread.uw.fpsimd_state;
 	unsigned int i;
 
 	if (!test_tsk_thread_flag(task, TIF_SVE))
@@ -910,7 +911,7 @@ void fpsimd_thread_switch(struct task_struct *next)
 		 * upon the next return to userland.
 		 */
 		if (__this_cpu_read(fpsimd_last_state.st) ==
-			&next->thread.fpsimd_state
+			&next->thread.uw.fpsimd_state
 		    && next->thread.fpsimd_cpu == smp_processor_id())
 			clear_tsk_thread_flag(next, TIF_FOREIGN_FPSTATE);
 		else
@@ -927,8 +928,8 @@ void fpsimd_flush_thread(void)
 
 	local_bh_disable();
 
-	memset(&current->thread.fpsimd_state, 0,
-	       sizeof(current->thread.fpsimd_state));
+	memset(&current->thread.uw.fpsimd_state, 0,
+	       sizeof(current->thread.uw.fpsimd_state));
 	fpsimd_flush_task_state(current);
 
 	if (system_supports_sve()) {
@@ -987,7 +988,7 @@ void fpsimd_preserve_current_state(void)
 
 /*
  * Like fpsimd_preserve_current_state(), but ensure that
- * current->thread.fpsimd_state is updated so that it can be copied to
+ * current->thread.uw.fpsimd_state is updated so that it can be copied to
  * the signal frame.
  */
 void fpsimd_signal_preserve_current_state(void)
@@ -1006,7 +1007,7 @@ static void fpsimd_bind_to_cpu(void)
 	struct fpsimd_last_state_struct *last =
 		this_cpu_ptr(&fpsimd_last_state);
 
-	last->st = &current->thread.fpsimd_state;
+	last->st = &current->thread.uw.fpsimd_state;
 	last->sve_in_use = test_thread_flag(TIF_SVE);
 	current->thread.fpsimd_cpu = smp_processor_id();
 }
@@ -1043,7 +1044,7 @@ void fpsimd_update_current_state(struct user_fpsimd_state const *state)
 
 	local_bh_disable();
 
-	current->thread.fpsimd_state = *state;
+	current->thread.uw.fpsimd_state = *state;
 	if (system_supports_sve() && test_thread_flag(TIF_SVE))
 		fpsimd_to_sve(current);