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Date: Sat, 8 Jun 2024 09:31:28 +0200
From: Ingo Molnar <mingo@...nel.org>
To: linux-kernel@...r.kernel.org
Cc: Andy Lutomirski <luto@...capital.net>,
Andrew Morton <akpm@...ux-foundation.org>,
Dave Hansen <dave@...1.net>,
Peter Zijlstra <peterz@...radead.org>,
Borislav Petkov <bp@...en8.de>,
Brian Gerst <brgerst@...il.com>,
"H . Peter Anvin" <hpa@...or.com>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Oleg Nesterov <oleg@...hat.com>,
Thomas Gleixner <tglx@...utronix.de>,
Uros Bizjak <ubizjak@...il.com>
Subject: [PATCH 3/9] x86/fpu: Make task_struct::thread constant size
Turn thread.fpu into a pointer. Since most FPU code internals work by passing
around the FPU pointer already, the code generation impact is small.
This allows us to remove the old kludge of task_struct being variable size:
struct task_struct {
...
/*
* New fields for task_struct should be added above here, so that
* they are included in the randomized portion of task_struct.
*/
randomized_struct_fields_end
/* CPU-specific state of this task: */
struct thread_struct thread;
/*
* WARNING: on x86, 'thread_struct' contains a variable-sized
* structure. It *MUST* be at the end of 'task_struct'.
*
* Do not put anything below here!
*/
};
... which creates a number of problems, such as requiring thread_struct to be
the last member of the struct - not allowing it to be struct-randomized, etc.
But the primary motivation is to allow the decoupling of task_struct from
hardware details (<asm/processor.h> in particular), and to eventually allow
the per-task infrastructure:
DECLARE_PER_TASK(type, name);
...
per_task(current, name) = val;
... which requires task_struct to be a constant size struct.
The fpu_thread_struct_whitelist() quirk to hardened usercopy can be removed,
now that the FPU structure is not embedded in the task struct anymore, which
reduces text footprint a bit.
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Cc: Andy Lutomirski <luto@...nel.org>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Fenghua Yu <fenghua.yu@...el.com>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Oleg Nesterov <oleg@...hat.com>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Uros Bizjak <ubizjak@...il.com>
Link: https://lore.kernel.org/r/20240605083557.2051480-2-mingo@kernel.org
---
arch/x86/include/asm/processor.h | 20 +++++++++-----------
arch/x86/kernel/fpu/core.c | 23 ++++++++++++-----------
arch/x86/kernel/fpu/init.c | 19 ++++++++++++-------
arch/x86/kernel/process.c | 2 +-
include/linux/sched.h | 13 +++----------
5 files changed, 37 insertions(+), 40 deletions(-)
diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index 35aa8f652964..64509c7f26c8 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -504,21 +504,19 @@ struct thread_struct {
#endif
/* Floating point and extended processor state */
- struct fpu fpu;
- /*
- * WARNING: 'fpu' is dynamically-sized. It *MUST* be at
- * the end.
- */
+ struct fpu *fpu;
};
-#define x86_task_fpu(task) (&(task)->thread.fpu)
+#define x86_task_fpu(task) ((task)->thread.fpu)
-extern void fpu_thread_struct_whitelist(unsigned long *offset, unsigned long *size);
-
-static inline void arch_thread_struct_whitelist(unsigned long *offset,
- unsigned long *size)
+/*
+ * X86 doesn't need any embedded-FPU-struct quirks:
+ */
+static inline void
+arch_thread_struct_whitelist(unsigned long *offset, unsigned long *size)
{
- fpu_thread_struct_whitelist(offset, size);
+ *offset = 0;
+ *size = 0;
}
static inline void
diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index ca6745f8ac2a..f0c4367804b3 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -584,8 +584,19 @@ static int update_fpu_shstk(struct task_struct *dst, unsigned long ssp)
int fpu_clone(struct task_struct *dst, unsigned long clone_flags, bool minimal,
unsigned long ssp)
{
+ /*
+ * We allocate the new FPU structure right after the end of the task struct.
+ * task allocation size already took this into account.
+ *
+ * This is safe because task_struct size is a multiple of cacheline size.
+ */
struct fpu *src_fpu = x86_task_fpu(current);
- struct fpu *dst_fpu = x86_task_fpu(dst);
+ struct fpu *dst_fpu = (void *)dst + sizeof(*dst);
+
+ BUILD_BUG_ON(sizeof(*dst) % SMP_CACHE_BYTES != 0);
+ BUG_ON(!src_fpu);
+
+ dst->thread.fpu = dst_fpu;
/* The new task's FPU state cannot be valid in the hardware. */
dst_fpu->last_cpu = -1;
@@ -654,16 +665,6 @@ int fpu_clone(struct task_struct *dst, unsigned long clone_flags, bool minimal,
return 0;
}
-/*
- * Whitelist the FPU register state embedded into task_struct for hardened
- * usercopy.
- */
-void fpu_thread_struct_whitelist(unsigned long *offset, unsigned long *size)
-{
- *offset = offsetof(struct thread_struct, fpu.__fpstate.regs);
- *size = fpu_kernel_cfg.default_size;
-}
-
/*
* Drops current FPU state: deactivates the fpregs and
* the fpstate. NOTE: it still leaves previous contents
diff --git a/arch/x86/kernel/fpu/init.c b/arch/x86/kernel/fpu/init.c
index ad5cb2943d37..4e8d37b5a90b 100644
--- a/arch/x86/kernel/fpu/init.c
+++ b/arch/x86/kernel/fpu/init.c
@@ -71,8 +71,17 @@ static bool __init fpu__probe_without_cpuid(void)
return fsw == 0 && (fcw & 0x103f) == 0x003f;
}
+static struct fpu x86_init_fpu __read_mostly;
+
static void __init fpu__init_system_early_generic(void)
{
+ int this_cpu = smp_processor_id();
+
+ fpstate_reset(&x86_init_fpu);
+ current->thread.fpu = &x86_init_fpu;
+ per_cpu(fpu_fpregs_owner_ctx, this_cpu) = &x86_init_fpu;
+ x86_init_fpu.last_cpu = this_cpu;
+
if (!boot_cpu_has(X86_FEATURE_CPUID) &&
!test_bit(X86_FEATURE_FPU, (unsigned long *)cpu_caps_cleared)) {
if (fpu__probe_without_cpuid())
@@ -150,6 +159,8 @@ static void __init fpu__init_task_struct_size(void)
{
int task_size = sizeof(struct task_struct);
+ task_size += sizeof(struct fpu);
+
/*
* Subtract off the static size of the register state.
* It potentially has a bunch of padding.
@@ -164,14 +175,9 @@ static void __init fpu__init_task_struct_size(void)
/*
* We dynamically size 'struct fpu', so we require that
- * it be at the end of 'thread_struct' and that
- * 'thread_struct' be at the end of 'task_struct'. If
- * you hit a compile error here, check the structure to
- * see if something got added to the end.
+ * 'state' be at the end of 'it:
*/
CHECK_MEMBER_AT_END_OF(struct fpu, __fpstate);
- CHECK_MEMBER_AT_END_OF(struct thread_struct, fpu);
- CHECK_MEMBER_AT_END_OF(struct task_struct, thread);
arch_task_struct_size = task_size;
}
@@ -213,7 +219,6 @@ static void __init fpu__init_system_xstate_size_legacy(void)
*/
void __init fpu__init_system(void)
{
- fpstate_reset(x86_task_fpu(current));
fpu__init_system_early_generic();
/*
diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index adfeefd6375a..5bb73bc0e31a 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -97,7 +97,7 @@ int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
dst->thread.vm86 = NULL;
#endif
/* Drop the copied pointer to current's fpstate */
- x86_task_fpu(dst)->fpstate = NULL;
+ dst->thread.fpu = NULL;
return 0;
}
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 61591ac6eab6..215a7380e41c 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1554,21 +1554,14 @@ struct task_struct {
struct user_event_mm *user_event_mm;
#endif
+ /* CPU-specific state of this task: */
+ struct thread_struct thread;
+
/*
* New fields for task_struct should be added above here, so that
* they are included in the randomized portion of task_struct.
*/
randomized_struct_fields_end
-
- /* CPU-specific state of this task: */
- struct thread_struct thread;
-
- /*
- * WARNING: on x86, 'thread_struct' contains a variable-sized
- * structure. It *MUST* be at the end of 'task_struct'.
- *
- * Do not put anything below here!
- */
};
#define TASK_REPORT_IDLE (TASK_REPORT + 1)
--
2.43.0
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