[PATCH v7 1/4] syscalls: Restore address limit after a syscall
flat view
From: mingo@kernel.org (Ingo Molnar)
Date: 2017-04-25 06:33:27
Also in:
linux-api, linux-s390, lkml
* Thomas Garnier [off-list ref] wrote:
This patch ensures a syscall does not return to user-mode with a kernel address limit. If that happened, a process can corrupt kernel-mode memory and elevate privileges.
Don't start changelogs with 'This patch' - it's obvious that we are talking about this patch. Writing: Ensure that a syscall does not return to user-mode with a kernel address limit. If that happens, a process can corrupt kernel-mode memory and elevate privileges. also note the spelling fix I did. (There's another spelling error elsewhere in this changelog as well.) Please read changelogs!
For example, it would mitigation this bug: - https://bugs.chromium.org/p/project-zero/issues/detail?id=990 The CONFIG_ARCH_NO_SYSCALL_VERIFY_PRE_USERMODE_STATE option is also added so each architecture can optimize this change.
As I pointed it out in my previous reply this Kconfig name is awfully long - but it should have been obvious when this changelog was written ...
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Signed-off-by: Thomas Garnier <redacted> Tested-by: Kees Cook <redacted> --- Based on next-20170410 --- arch/s390/Kconfig | 1 + include/linux/syscalls.h | 26 +++++++++++++++++++++++++- init/Kconfig | 6 ++++++ kernel/sys.c | 13 +++++++++++++ 4 files changed, 45 insertions(+), 1 deletion(-)diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig index d25435d94b6e..489a0cc6e46b 100644 --- a/arch/s390/Kconfig +++ b/arch/s390/Kconfig@@ -103,6 +103,7 @@ config S390 select ARCH_INLINE_WRITE_UNLOCK_BH select ARCH_INLINE_WRITE_UNLOCK_IRQ select ARCH_INLINE_WRITE_UNLOCK_IRQRESTORE + select ARCH_NO_SYSCALL_VERIFY_PRE_USERMODE_STATE select ARCH_SAVE_PAGE_KEYS if HIBERNATION select ARCH_SUPPORTS_ATOMIC_RMW select ARCH_SUPPORTS_DEFERRED_STRUCT_PAGE_INITdiff --git a/include/linux/syscalls.h b/include/linux/syscalls.h index 980c3c9b06f8..801a7a74fe28 100644 --- a/include/linux/syscalls.h +++ b/include/linux/syscalls.h@@ -191,6 +191,27 @@ extern struct trace_event_functions exit_syscall_print_funcs; SYSCALL_METADATA(sname, x, __VA_ARGS__) \ __SYSCALL_DEFINEx(x, sname, __VA_ARGS__) + +/* + * Called before coming back to user-mode. Returning to user-mode with an + * address limit different than USER_DS can allow to overwrite kernel memory. + */ +static inline void verify_pre_usermode_state(void) { + BUG_ON(!segment_eq(get_fs(), USER_DS)); +}
Non-standard coding style.
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+ +#ifndef CONFIG_ARCH_NO_SYSCALL_VERIFY_PRE_USERMODE_STATE +#define __CHECK_USER_CALLER() \ + bool user_caller = segment_eq(get_fs(), USER_DS) +#define __VERIFY_PRE_USERMODE_STATE() \ + if (user_caller) verify_pre_usermode_state() +#else +#define __CHECK_USER_CALLER() +#define __VERIFY_PRE_USERMODE_STATE() +asmlinkage void address_limit_check_failed(void); +#endif + + #define __PROTECT(...) asmlinkage_protect(__VA_ARGS__) #define __SYSCALL_DEFINEx(x, name, ...) \ asmlinkage long sys##name(__MAP(x,__SC_DECL,__VA_ARGS__)) \@@ -199,7 +220,10 @@ extern struct trace_event_functions exit_syscall_print_funcs; asmlinkage long SyS##name(__MAP(x,__SC_LONG,__VA_ARGS__)); \ asmlinkage long SyS##name(__MAP(x,__SC_LONG,__VA_ARGS__)) \ { \ - long ret = SYSC##name(__MAP(x,__SC_CAST,__VA_ARGS__)); \ + long ret; \ + __CHECK_USER_CALLER(); \ + ret = SYSC##name(__MAP(x,__SC_CAST,__VA_ARGS__)); \ + __VERIFY_PRE_USERMODE_STATE(); \ __MAP(x,__SC_TEST,__VA_ARGS__); \ __PROTECT(x, ret,__MAP(x,__SC_ARGS,__VA_ARGS__)); \ return ret; \
BTW., the '__VERIFY_PRE_USERMODE_STATE()' name is highly misleading: the 'pre' prefix suggests that this is done before a system call - while it's done afterwards. The solution is to not try to specify the exact call placement in the name, just describe the functionality (and harmonize along the common prefix).
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+config ARCH_NO_SYSCALL_VERIFY_PRE_USERMODE_STATE + bool + help + Disable the generic pre-usermode state verification. Allow each + architecture to optimize how and when the verification is done. +
Please name the Kconfig symbols something like this: CONFIG_ADDR_LIMIT_CHECK CONFIG_ADDR_LIMIT_CHECK_ARCH or so, which tells us whether the check is done by the architecture code, without breaking the col80 limit with a single Kconfig name. BTW:
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+#ifdef CONFIG_ARCH_NO_SYSCALL_VERIFY_PRE_USERMODE_STATE +/* + * This function is called when an architecture specific implementation detected + * an invalid address limit. The generic user-mode state checker will finish on + * the appropriate BUG_ON. + */ +asmlinkage void address_limit_check_failed(void) +{ + verify_pre_usermode_state(); + panic("address_limit_check_failed called with a valid user-mode state");
It's very unconstructive to unconditionally panic the system, just because some kernel code leaked the address limit! Do a warn-once printout and kill the current task (i.e. don't continue execution), but don't crash everything else! Thanks, Ingo