[PATCH v5 3/7] arm64: uaccess: Add additional userspace GCS accessors
From: Jeremy Linton <hidden>
Date: 2025-08-11 14:10:27
Also in:
linux-arm-kernel, linux-perf-users, lkml
Subsystem:
arm64 port (aarch64 architecture), the rest · Maintainers:
Catalin Marinas, Will Deacon, Linus Torvalds
Uprobes need more advanced read, push, and pop userspace GCS functionality. Implement those features using the existing gcsstr() and copy_from_user(). Its important to note that GCS pages can be read by normal instructions, but the hardware validates that pages used by GCS specific operations, have a GCS privilege set. We aren't validating this in load_user_gcs because it requires stabilizing the VMA over the read which may fault. Signed-off-by: Jeremy Linton <redacted> Reviewed-by: Catalin Marinas <catalin.marinas@arm.com> --- arch/arm64/include/asm/gcs.h | 54 ++++++++++++++++++++++++++++++++++++ 1 file changed, 54 insertions(+)
diff --git a/arch/arm64/include/asm/gcs.h b/arch/arm64/include/asm/gcs.h
index e3b360c9dba4..bd9ff1dedcd9 100644
--- a/arch/arm64/include/asm/gcs.h
+++ b/arch/arm64/include/asm/gcs.h@@ -116,6 +116,47 @@ static inline void put_user_gcs(unsigned long val, unsigned long __user *addr, uaccess_ttbr0_disable(); } +static inline void push_user_gcs(unsigned long val, int *err) +{ + u64 gcspr = read_sysreg_s(SYS_GCSPR_EL0); + + gcspr -= sizeof(u64); + put_user_gcs(val, (unsigned long __user *)gcspr, err); + if (!*err) + write_sysreg_s(gcspr, SYS_GCSPR_EL0); +} + +/* + * Unlike put/push_user_gcs() above, get/pop_user_gsc() doesn't + * validate the GCS permission is set on the page being read. This + * differs from how the hardware works when it consumes data stored at + * GCSPR. Callers should assure this is acceptable. + */ +static inline u64 get_user_gcs(unsigned long __user *addr, int *err) +{ + unsigned long ret; + u64 load = 0; + + /* Ensure previous GCS operation are visible before we read the page */ + gcsb_dsync(); + ret = copy_from_user(&load, addr, sizeof(load)); + if (ret != 0) + *err = ret; + return load; +} + +static inline u64 pop_user_gcs(int *err) +{ + u64 gcspr = read_sysreg_s(SYS_GCSPR_EL0); + u64 read_val; + + read_val = get_user_gcs((unsigned long __user *)gcspr, err); + if (!*err) + write_sysreg_s(gcspr + sizeof(u64), SYS_GCSPR_EL0); + + return read_val; +} + #else static inline bool task_gcs_el0_enabled(struct task_struct *task)
@@ -126,6 +167,10 @@ static inline bool task_gcs_el0_enabled(struct task_struct *task) static inline void gcs_set_el0_mode(struct task_struct *task) { } static inline void gcs_free(struct task_struct *task) { } static inline void gcs_preserve_current_state(void) { } +static inline void put_user_gcs(unsigned long val, unsigned long __user *addr, + int *err) { } +static inline void push_user_gcs(unsigned long val, int *err) { } + static inline unsigned long gcs_alloc_thread_stack(struct task_struct *tsk, const struct kernel_clone_args *args) {
@@ -136,6 +181,15 @@ static inline int gcs_check_locked(struct task_struct *task, { return 0; } +static inline u64 load_user_gcs(unsigned long __user *addr, int *err) +{ + *err = -EFAULT; + return 0; +} +static inline u64 pop_user_gcs(int *err) +{ + return 0; +} #endif
--
2.50.1