Commit 030e8a40 authored by Kevin Brodsky's avatar Kevin Brodsky Committed by Catalin Marinas
Browse files

arm64: signal: Preserve POR_EL0 if poe_context is missing 



Commit 2e8a1ace ("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: 2e8a1ace ("arm64: signal: Improve POR_EL0 handling to avoid uaccess failures")
Cc: <stable@vger.kernel.org>
Reported-by: default avatarWill Deacon <will@kernel.org>
Signed-off-by: default avatarKevin Brodsky <kevin.brodsky@arm.com>
Signed-off-by: default avatarCatalin Marinas <catalin.marinas@arm.com>
parent 4d8e74ad

arch/arm64/kernel/signal.c

+43 −11
Original line number Diff line number Diff line
@@ -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();