[PATCH v2 1/5] arm64: signal: Preserve POR_EL0 if poe_context is missing

[Kevin Brodsky <kevin.brodsky@arm.com>]

Commit 2e8a1acea859 ("arm64: signal: Improve POR_EL0 handling to
avoid uaccess failures") delayed the write to POR_EL0 in
rt_sigreturn to avoid spurious uaccess failures. This change however
relies on the poe_context frame record being present: on a system
supporting POE, calling sigreturn without a poe_context record now
results in writing arbitrary data from the kernel stack into POR_EL0.

Fix this by adding a __valid_fields member to struct
user_access_state, and zeroing the struct on allocation.
restore_poe_context() then indicates that the por_el0 field is valid
by setting the corresponding bit in __valid_fields, and
restore_user_access_state() only touches POR_EL0 if there is a valid
value to set it to. This is in line with how POR_EL0 was originally
handled; all frame records are currently optional, except
fpsimd_context.

To ensure that __valid_fields is kept in sync, fields (currently
just por_el0) are now accessed via accessors and prefixed with __ to
discourage direct access.

Fixes: 2e8a1acea859 ("arm64: signal: Improve POR_EL0 handling to avoid uaccess failures")
Reported-by: Will Deacon <will@kernel.org>
Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
---
 arch/arm64/kernel/signal.c | 54 ++++++++++++++++++++++++++++++++++++----------
 1 file changed, 43 insertions(+), 11 deletions(-)

diff --git a/arch/arm64/kernel/signal.c b/arch/arm64/kernel/signal.c
index 08ffc5a5aea4..38e6fa204c17 100644
--- a/arch/arm64/kernel/signal.c
+++ b/arch/arm64/kernel/signal.c
@@ -67,6 +67,9 @@ struct rt_sigframe_user_layout {
 	unsigned long end_offset;
 };
 
+#define TERMINATOR_SIZE round_up(sizeof(struct _aarch64_ctx), 16)
+#define EXTRA_CONTEXT_SIZE round_up(sizeof(struct extra_context), 16)
+
 /*
  * Holds any EL0-controlled state that influences unprivileged memory accesses.
  * This includes both accesses done in userspace and uaccess done in the kernel.
@@ -74,13 +77,35 @@ struct rt_sigframe_user_layout {
  * This state needs to be carefully managed to ensure that it doesn't cause
  * uaccess to fail when setting up the signal frame, and the signal handler
  * itself also expects a well-defined state when entered.
+ *
+ * The struct should be zero-initialised. Its members should only be accessed
+ * via the accessors below. __valid_fields tracks which of the fields are valid
+ * (have been set to some value).
  */
 struct user_access_state {
-	u64 por_el0;
+	unsigned int __valid_fields;
+	u64 __por_el0;
 };
 
-#define TERMINATOR_SIZE round_up(sizeof(struct _aarch64_ctx), 16)
-#define EXTRA_CONTEXT_SIZE round_up(sizeof(struct extra_context), 16)
+#define UA_STATE_HAS_POR_EL0	BIT(0)
+
+static void set_ua_state_por_el0(struct user_access_state *ua_state,
+				 u64 por_el0)
+{
+	ua_state->__por_el0 = por_el0;
+	ua_state->__valid_fields |= UA_STATE_HAS_POR_EL0;
+}
+
+static int get_ua_state_por_el0(const struct user_access_state *ua_state,
+				u64 *por_el0)
+{
+	if (ua_state->__valid_fields & UA_STATE_HAS_POR_EL0) {
+		*por_el0 = ua_state->__por_el0;
+		return 0;
+	}
+
+	return -ENOENT;
+}
 
 /*
  * Save the user access state into ua_state and reset it to disable any
@@ -94,7 +119,7 @@ static void save_reset_user_access_state(struct user_access_state *ua_state)
 		for (int pkey = 0; pkey < arch_max_pkey(); pkey++)
 			por_enable_all |= POR_ELx_PERM_PREP(pkey, POE_RWX);
 
-		ua_state->por_el0 = read_sysreg_s(SYS_POR_EL0);
+		set_ua_state_por_el0(ua_state, read_sysreg_s(SYS_POR_EL0));
 		write_sysreg_s(por_enable_all, SYS_POR_EL0);
 		/*
 		 * No ISB required as we can tolerate spurious Overlay faults -
@@ -122,8 +147,10 @@ static void set_handler_user_access_state(void)
  */
 static void restore_user_access_state(const struct user_access_state *ua_state)
 {
-	if (system_supports_poe())
-		write_sysreg_s(ua_state->por_el0, SYS_POR_EL0);
+	u64 por_el0;
+
+	if (get_ua_state_por_el0(ua_state, &por_el0) == 0)
+		write_sysreg_s(por_el0, SYS_POR_EL0);
 }
 
 static void init_user_layout(struct rt_sigframe_user_layout *user)
@@ -333,11 +360,16 @@ static int restore_fpmr_context(struct user_ctxs *user)
 static int preserve_poe_context(struct poe_context __user *ctx,
 				const struct user_access_state *ua_state)
 {
-	int err = 0;
+	int err;
+	u64 por_el0;
+
+	err = get_ua_state_por_el0(ua_state, &por_el0);
+	if (WARN_ON_ONCE(err))
+		return err;
 
 	__put_user_error(POE_MAGIC, &ctx->head.magic, err);
 	__put_user_error(sizeof(*ctx), &ctx->head.size, err);
-	__put_user_error(ua_state->por_el0, &ctx->por_el0, err);
+	__put_user_error(por_el0, &ctx->por_el0, err);
 
 	return err;
 }
@@ -353,7 +385,7 @@ static int restore_poe_context(struct user_ctxs *user,
 
 	__get_user_error(por_el0, &(user->poe->por_el0), err);
 	if (!err)
-		ua_state->por_el0 = por_el0;
+		set_ua_state_por_el0(ua_state, por_el0);
 
 	return err;
 }
@@ -1095,7 +1127,7 @@ SYSCALL_DEFINE0(rt_sigreturn)
 {
 	struct pt_regs *regs = current_pt_regs();
 	struct rt_sigframe __user *frame;
-	struct user_access_state ua_state;
+	struct user_access_state ua_state = {};
 
 	/* Always make any pending restarted system calls return -EINTR */
 	current->restart_block.fn = do_no_restart_syscall;
@@ -1507,7 +1539,7 @@ static int setup_rt_frame(int usig, struct ksignal *ksig, sigset_t *set,
 {
 	struct rt_sigframe_user_layout user;
 	struct rt_sigframe __user *frame;
-	struct user_access_state ua_state;
+	struct user_access_state ua_state = {};
 	int err = 0;
 
 	fpsimd_save_and_flush_current_state();

-- 
2.51.2