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arm64: signal: Preserve POR_EL0 if poe_context is missing

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Commit 2e8a1acea8 ("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: 2e8a1acea8 ("arm64: signal: Improve POR_EL0 handling to avoid uaccess failures")
Cc: <stable@vger.kernel.org>
Reported-by: Will Deacon <will@kernel.org>
Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
This commit is contained in:
Kevin Brodsky
2026-04-27 13:03:33 +01:00
committed by Catalin Marinas
parent 4d8e74ad45
commit 030e8a40ff
1 changed files with 43 additions and 11 deletions
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54
arch/arm64/kernel/signal.c
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@@ -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();