Re: [RFC PATCH 6/8] preempt/dynamic: Provide preempt_schedule[_notrace]() static calls

From: Peter Zijlstra
Date: Fri Feb 05 2021 - 13:02:23 EST

On Wed, Feb 03, 2021 at 03:04:23PM +0100, Peter Zijlstra wrote:
> Fair enough I suppose. I'll slap a changelog and your SoB on it and I
> suppose I'll got commit the whole lot. Then we can forget about it
> again.

FWIW, the whole thing looks like this..

---
Subject: static_call: Allow module use without exposing static_call_key
From: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Date: Wed, 27 Jan 2021 17:18:37 -0600

From: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>

When exporting static_call_key; with EXPORT_STATIC_CALL*(), the module
can use static_call_update() to change the function called. This is
not desirable in general.

Not exporting static_call_key however also disallows usage of
static_call(), since objtool needs the key to construct the
static_call_site.

Solve this by allowing objtool to create the static_call_site using
the trampoline address when it builds a module and cannot find the
static_call_key symbol. The module loader will then try and map the
trampole back to a key before it constructs the normal sites list.

Doing this requires a trampoline -> key associsation, so add another
magic section that keeps those.

Originally-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20210127231837.ifddpn7rhwdaepiu@treble
---
arch/x86/include/asm/static_call.h | 7 ++++
include/asm-generic/vmlinux.lds.h | 5 ++
include/linux/static_call.h | 22 +++++++++++-
include/linux/static_call_types.h | 27 ++++++++++++++-
kernel/static_call.c | 55 ++++++++++++++++++++++++++++++--
tools/include/linux/static_call_types.h | 27 ++++++++++++++-
tools/objtool/check.c | 17 ++++++++-
7 files changed, 149 insertions(+), 11 deletions(-)

--- a/arch/x86/include/asm/static_call.h
+++ b/arch/x86/include/asm/static_call.h
@@ -37,4 +37,11 @@
#define ARCH_DEFINE_STATIC_CALL_NULL_TRAMP(name) \
__ARCH_DEFINE_STATIC_CALL_TRAMP(name, "ret; nop; nop; nop; nop")

+
+#define ARCH_ADD_TRAMP_KEY(name) \
+ asm(".pushsection .static_call_tramp_key, \"a\" \n" \
+ ".long " STATIC_CALL_TRAMP_STR(name) " - . \n" \
+ ".long " STATIC_CALL_KEY_STR(name) " - . \n" \
+ ".popsection \n")
+
#endif /* _ASM_STATIC_CALL_H */
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -393,7 +393,10 @@
. = ALIGN(8); \
__start_static_call_sites = .; \
KEEP(*(.static_call_sites)) \
- __stop_static_call_sites = .;
+ __stop_static_call_sites = .; \
+ __start_static_call_tramp_key = .; \
+ KEEP(*(.static_call_tramp_key)) \
+ __stop_static_call_tramp_key = .;

/*
* Allow architectures to handle ro_after_init data on their
--- a/include/linux/static_call.h
+++ b/include/linux/static_call.h
@@ -138,6 +138,12 @@ struct static_call_key {
};
};

+/* For finding the key associated with a trampoline */
+struct static_call_tramp_key {
+ s32 tramp;
+ s32 key;
+};
+
extern void __static_call_update(struct static_call_key *key, void *tramp, void *func);
extern int static_call_mod_init(struct module *mod);
extern int static_call_text_reserved(void *start, void *end);
@@ -165,11 +171,18 @@ extern long __static_call_return0(void);
#define EXPORT_STATIC_CALL(name) \
EXPORT_SYMBOL(STATIC_CALL_KEY(name)); \
EXPORT_SYMBOL(STATIC_CALL_TRAMP(name))
-
#define EXPORT_STATIC_CALL_GPL(name) \
EXPORT_SYMBOL_GPL(STATIC_CALL_KEY(name)); \
EXPORT_SYMBOL_GPL(STATIC_CALL_TRAMP(name))

+/* Leave the key unexported, so modules can't change static call targets: */
+#define EXPORT_STATIC_CALL_TRAMP(name) \
+ EXPORT_SYMBOL(STATIC_CALL_TRAMP(name)); \
+ ARCH_ADD_TRAMP_KEY(name)
+#define EXPORT_STATIC_CALL_TRAMP_GPL(name) \
+ EXPORT_SYMBOL_GPL(STATIC_CALL_TRAMP(name)); \
+ ARCH_ADD_TRAMP_KEY(name)
+
#elif defined(CONFIG_HAVE_STATIC_CALL)

static inline int static_call_init(void) { return 0; }
@@ -216,11 +229,16 @@ static inline long __static_call_return0
#define EXPORT_STATIC_CALL(name) \
EXPORT_SYMBOL(STATIC_CALL_KEY(name)); \
EXPORT_SYMBOL(STATIC_CALL_TRAMP(name))
-
#define EXPORT_STATIC_CALL_GPL(name) \
EXPORT_SYMBOL_GPL(STATIC_CALL_KEY(name)); \
EXPORT_SYMBOL_GPL(STATIC_CALL_TRAMP(name))

+/* Leave the key unexported, so modules can't change static call targets: */
+#define EXPORT_STATIC_CALL_TRAMP(name) \
+ EXPORT_SYMBOL(STATIC_CALL_TRAMP(name))
+#define EXPORT_STATIC_CALL_TRAMP_GPL(name) \
+ EXPORT_SYMBOL_GPL(STATIC_CALL_TRAMP(name))
+
#else /* Generic implementation */

static inline int static_call_init(void) { return 0; }
--- a/include/linux/static_call_types.h
+++ b/include/linux/static_call_types.h
@@ -10,6 +10,7 @@
#define STATIC_CALL_KEY_PREFIX_STR __stringify(STATIC_CALL_KEY_PREFIX)
#define STATIC_CALL_KEY_PREFIX_LEN (sizeof(STATIC_CALL_KEY_PREFIX_STR) - 1)
#define STATIC_CALL_KEY(name) __PASTE(STATIC_CALL_KEY_PREFIX, name)
+#define STATIC_CALL_KEY_STR(name) __stringify(STATIC_CALL_KEY(name))

#define STATIC_CALL_TRAMP_PREFIX __SCT__
#define STATIC_CALL_TRAMP_PREFIX_STR __stringify(STATIC_CALL_TRAMP_PREFIX)
@@ -39,17 +40,39 @@ struct static_call_site {

#ifdef CONFIG_HAVE_STATIC_CALL

+#define __raw_static_call(name) (&STATIC_CALL_TRAMP(name))
+
+#ifdef CONFIG_HAVE_STATIC_CALL_INLINE
+
/*
* __ADDRESSABLE() is used to ensure the key symbol doesn't get stripped from
* the symbol table so that objtool can reference it when it generates the
* .static_call_sites section.
*/
+#define __STATIC_CALL_ADDRESSABLE(name) \
+ __ADDRESSABLE(STATIC_CALL_KEY(name))
+
#define __static_call(name) \
({ \
- __ADDRESSABLE(STATIC_CALL_KEY(name)); \
- &STATIC_CALL_TRAMP(name); \
+ __STATIC_CALL_ADDRESSABLE(name); \
+ __raw_static_call(name); \
})

+#else /* !CONFIG_HAVE_STATIC_CALL_INLINE */
+
+#define __STATIC_CALL_ADDRESSABLE(name)
+#define __static_call(name) __raw_static_call(name)
+
+#endif /* CONFIG_HAVE_STATIC_CALL_INLINE */
+
+#ifdef MODULE
+#define __STATIC_CALL_MOD_ADDRESSABLE(name)
+#define static_call_mod(name) __raw_static_call(name)
+#else
+#define __STATIC_CALL_MOD_ADDRESSABLE(name) __STATIC_CALL_ADDRESSABLE(name)
+#define static_call_mod(name) __static_call(name)
+#endif
+
#define static_call(name) __static_call(name)

#else
--- a/kernel/static_call.c
+++ b/kernel/static_call.c
@@ -12,6 +12,8 @@

extern struct static_call_site __start_static_call_sites[],
__stop_static_call_sites[];
+extern struct static_call_tramp_key __start_static_call_tramp_key[],
+ __stop_static_call_tramp_key[];

static bool static_call_initialized;

@@ -323,10 +325,59 @@ static int __static_call_mod_text_reserv
return ret;
}

+static unsigned long tramp_key_lookup(unsigned long addr)
+{
+ struct static_call_tramp_key *start = __start_static_call_tramp_key;
+ struct static_call_tramp_key *stop = __stop_static_call_tramp_key;
+ struct static_call_tramp_key *tramp_key;
+
+ for (tramp_key = start; tramp_key != stop; tramp_key++) {
+ unsigned long tramp;
+
+ tramp = (long)tramp_key->tramp + (long)&tramp_key->tramp;
+ if (tramp == addr)
+ return (long)tramp_key->key + (long)&tramp_key->key;
+ }
+
+ return 0;
+}
+
static int static_call_add_module(struct module *mod)
{
- return __static_call_init(mod, mod->static_call_sites,
- mod->static_call_sites + mod->num_static_call_sites);
+ struct static_call_site *start = mod->static_call_sites;
+ struct static_call_site *stop = start + mod->num_static_call_sites;
+ struct static_call_site *site;
+
+ for (site = start; site != stop; site++) {
+ unsigned long addr = (unsigned long)static_call_key(site);
+ unsigned long key;
+
+ /*
+ * Is the key is exported, 'addr' points to the key, which
+ * means modules are allowed to call static_call_update() on
+ * it.
+ *
+ * Otherwise, the key isn't exported, and 'addr' points to the
+ * trampoline so we need to lookup the key.
+ *
+ * We go through this dance to prevent crazy modules from
+ * abusing sensitive static calls.
+ */
+ if (!kernel_text_address(addr))
+ continue;
+
+ key = tramp_key_lookup(addr);
+ if (!key) {
+ pr_warn("Failed to fixup __raw_static_call() usage at: %ps\n",
+ static_call_addr(site));
+ return -EINVAL;
+ }
+
+ site->key = (key - (long)&site->key) |
+ (site->key & STATIC_CALL_SITE_FLAGS);
+ }
+
+ return __static_call_init(mod, start, stop);
}

static void static_call_del_module(struct module *mod)
--- a/tools/include/linux/static_call_types.h
+++ b/tools/include/linux/static_call_types.h
@@ -10,6 +10,7 @@
#define STATIC_CALL_KEY_PREFIX_STR __stringify(STATIC_CALL_KEY_PREFIX)
#define STATIC_CALL_KEY_PREFIX_LEN (sizeof(STATIC_CALL_KEY_PREFIX_STR) - 1)
#define STATIC_CALL_KEY(name) __PASTE(STATIC_CALL_KEY_PREFIX, name)
+#define STATIC_CALL_KEY_STR(name) __stringify(STATIC_CALL_KEY(name))

#define STATIC_CALL_TRAMP_PREFIX __SCT__
#define STATIC_CALL_TRAMP_PREFIX_STR __stringify(STATIC_CALL_TRAMP_PREFIX)
@@ -39,17 +40,39 @@ struct static_call_site {

#ifdef CONFIG_HAVE_STATIC_CALL

+#define __raw_static_call(name) (&STATIC_CALL_TRAMP(name))
+
+#ifdef CONFIG_HAVE_STATIC_CALL_INLINE
+
/*
* __ADDRESSABLE() is used to ensure the key symbol doesn't get stripped from
* the symbol table so that objtool can reference it when it generates the
* .static_call_sites section.
*/
+#define __STATIC_CALL_ADDRESSABLE(name) \
+ __ADDRESSABLE(STATIC_CALL_KEY(name))
+
#define __static_call(name) \
({ \
- __ADDRESSABLE(STATIC_CALL_KEY(name)); \
- &STATIC_CALL_TRAMP(name); \
+ __STATIC_CALL_ADDRESSABLE(name); \
+ __raw_static_call(name); \
})

+#else /* !CONFIG_HAVE_STATIC_CALL_INLINE */
+
+#define __STATIC_CALL_ADDRESSABLE(name)
+#define __static_call(name) __raw_static_call(name)
+
+#endif /* CONFIG_HAVE_STATIC_CALL_INLINE */
+
+#ifdef MODULE
+#define __STATIC_CALL_MOD_ADDRESSABLE(name)
+#define static_call_mod(name) __raw_static_call(name)
+#else
+#define __STATIC_CALL_MOD_ADDRESSABLE(name) __STATIC_CALL_ADDRESSABLE(name)
+#define static_call_mod(name) __static_call(name)
+#endif
+
#define static_call(name) __static_call(name)

#else
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -507,8 +507,21 @@ static int create_static_call_sections(s

key_sym = find_symbol_by_name(file->elf, tmp);
if (!key_sym) {
- WARN("static_call: can't find static_call_key symbol: %s", tmp);
- return -1;
+ if (!module) {
+ WARN("static_call: can't find static_call_key symbol: %s", tmp);
+ return -1;
+ }
+
+ /*
+ * For modules(), the key might not be exported, which
+ * means the module can make static calls but isn't
+ * allowed to change them.
+ *
+ * In that case we temporarily set the key to be the
+ * trampoline address. This is fixed up in
+ * static_call_add_module().
+ */
+ key_sym = insn->call_dest;
}
free(key_name);