From: Johannes Berg johannes.berg@intel.com
commit adf9ae0d159d3dc94f58d788fc4757c8749ac0df upstream.
In commit 9f0b4807a44f ("um: rework userspace stubs to not hard-code stub location") I changed stub_segv_handler() to do a calculation with a pointer to a stack variable to find the data page that we're using for the stack and the rest of the data. This same commit was meant to do it as well for stub_clone_handler(), but the change inadvertently went into commit 84b2789d6115 ("um: separate child and parent errors in clone stub") instead.
This was reported to not be compiled correctly by gcc 5, causing the code to crash here. I'm not sure why, perhaps it's UB because the var isn't initialized? In any case, this trick always seemed bad, so just create a new inline function that does the calculation in assembly.
Reported-by: subashab@codeaurora.org Fixes: 9f0b4807a44f ("um: rework userspace stubs to not hard-code stub location") Fixes: 84b2789d6115 ("um: separate child and parent errors in clone stub") Signed-off-by: Johannes Berg johannes.berg@intel.com Signed-off-by: Richard Weinberger richard@nod.at Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- arch/um/kernel/skas/clone.c | 3 +-- arch/x86/um/shared/sysdep/stub_32.h | 12 ++++++++++++ arch/x86/um/shared/sysdep/stub_64.h | 12 ++++++++++++ arch/x86/um/stub_segv.c | 3 +-- 4 files changed, 26 insertions(+), 4 deletions(-)
--- a/arch/um/kernel/skas/clone.c +++ b/arch/um/kernel/skas/clone.c @@ -24,8 +24,7 @@ void __attribute__ ((__section__ (".__syscall_stub"))) stub_clone_handler(void) { - int stack; - struct stub_data *data = (void *) ((unsigned long)&stack & ~(UM_KERN_PAGE_SIZE - 1)); + struct stub_data *data = get_stub_page(); long err;
err = stub_syscall2(__NR_clone, CLONE_PARENT | CLONE_FILES | SIGCHLD, --- a/arch/x86/um/shared/sysdep/stub_32.h +++ b/arch/x86/um/shared/sysdep/stub_32.h @@ -101,4 +101,16 @@ static inline void remap_stack_and_trap( "memory"); }
+static __always_inline void *get_stub_page(void) +{ + unsigned long ret; + + asm volatile ( + "movl %%esp,%0 ;" + "andl %1,%0" + : "=a" (ret) + : "g" (~(UM_KERN_PAGE_SIZE - 1))); + + return (void *)ret; +} #endif --- a/arch/x86/um/shared/sysdep/stub_64.h +++ b/arch/x86/um/shared/sysdep/stub_64.h @@ -108,4 +108,16 @@ static inline void remap_stack_and_trap( __syscall_clobber, "r10", "r8", "r9"); }
+static __always_inline void *get_stub_page(void) +{ + unsigned long ret; + + asm volatile ( + "movq %%rsp,%0 ;" + "andq %1,%0" + : "=a" (ret) + : "g" (~(UM_KERN_PAGE_SIZE - 1))); + + return (void *)ret; +} #endif --- a/arch/x86/um/stub_segv.c +++ b/arch/x86/um/stub_segv.c @@ -11,9 +11,8 @@ void __attribute__ ((__section__ (".__syscall_stub"))) stub_segv_handler(int sig, siginfo_t *info, void *p) { - int stack; + struct faultinfo *f = get_stub_page(); ucontext_t *uc = p; - struct faultinfo *f = (void *)(((unsigned long)&stack) & ~(UM_KERN_PAGE_SIZE - 1));
GET_FAULTINFO_FROM_MC(*f, &uc->uc_mcontext); trap_myself();