arm64/sve: Better handle failure to allocate SVE register storage

Currently we "handle" failure to allocate the SVE register storage by
doing a BUG_ON() and hoping for the best. This is obviously not great and
the memory allocation failure will already be loud enough without the
BUG_ON(). As the comment says it is a corner case but let's try to do a bit
better, remove the BUG_ON() and add code to handle the failure in the
callers.

For the ptrace and signal code we can return -ENOMEM gracefully however
we have no real error reporting path available to us for the SVE access
trap so instead generate a SIGKILL if the allocation fails there. This
at least means that we won't try to soldier on and end up trying to
access the nonexistant state and while it's obviously not ideal for
userspace SIGKILL doesn't allow any handling so minimises the ABI
impact, making it easier to improve the interface later if we come up
with a better idea.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20210824153417.18371-1-broonie@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/fpsimd.c

@@ -520,12 +520,6 @@ void sve_alloc(struct task_struct *task)
 	/* This is a small allocation (maximum ~8KB) and Should Not Fail. */
 	task->thread.sve_state =
 		kzalloc(sve_state_size(task), GFP_KERNEL);
-
-	/*
-	 * If future SVE revisions can have larger vectors though,
-	 * this may cease to be true:
-	 */
-	BUG_ON(!task->thread.sve_state);
 }
 
 
@@ -945,6 +939,10 @@ void do_sve_acc(unsigned int esr, struct pt_regs *regs)
 	}
 
 	sve_alloc(current);
+	if (!current->thread.sve_state) {
+		force_sig(SIGKILL);
+		return;
+	}
 
 	get_cpu_fpsimd_context();
 

arch/arm64/kernel/ptrace.c

@@ -845,6 +845,11 @@ static int sve_set(struct task_struct *target,
 	}
 
 	sve_alloc(target);
+	if (!target->thread.sve_state) {
+		ret = -ENOMEM;
+		clear_tsk_thread_flag(target, TIF_SVE);
+		goto out;
+	}
 
 	/*
 	 * Ensure target->thread.sve_state is up to date with target's

arch/arm64/kernel/signal.c

@@ -289,6 +289,11 @@ static int restore_sve_fpsimd_context(struct user_ctxs *user)
 	/* From now, fpsimd_thread_switch() won't touch thread.sve_state */
 
 	sve_alloc(current);
+	if (!current->thread.sve_state) {
+		clear_thread_flag(TIF_SVE);
+		return -ENOMEM;
+	}
+
 	err = __copy_from_user(current->thread.sve_state,
 			       (char __user const *)user->sve +
 					SVE_SIG_REGS_OFFSET,