From acf46020012ccbca1172e9c7aeab399c950d9212 Mon Sep 17 00:00:00 2001 From: Dmitry Safonov Date: Fri, 18 May 2018 00:35:10 +0100 Subject: [PATCH] x86/mm: Drop TS_COMPAT on 64-bit exec() syscall The x86 mmap() code selects the mmap base for an allocation depending on the bitness of the syscall. For 64bit sycalls it select mm->mmap_base and for 32bit mm->mmap_compat_base. exec() calls mmap() which in turn uses in_compat_syscall() to check whether the mapping is for a 32bit or a 64bit task. The decision is made on the following criteria: ia32 child->thread.status & TS_COMPAT x32 child->pt_regs.orig_ax & __X32_SYSCALL_BIT ia64 !ia32 && !x32 __set_personality_x32() was dropping TS_COMPAT flag, but set_personality_64bit() has kept compat syscall flag making in_compat_syscall() return true during the first exec() syscall. Which in result has user-visible effects, mentioned by Alexey: 1) It breaks ASAN $ gcc -fsanitize=address wrap.c -o wrap-asan $ ./wrap32 ./wrap-asan true ==1217==Shadow memory range interleaves with an existing memory mapping. ASan cannot proceed correctly. ABORTING. ==1217==ASan shadow was supposed to be located in the [0x00007fff7000-0x10007fff7fff] range. ==1217==Process memory map follows: 0x000000400000-0x000000401000 /home/izbyshev/test/gcc/asan-exec-from-32bit/wrap-asan 0x000000600000-0x000000601000 /home/izbyshev/test/gcc/asan-exec-from-32bit/wrap-asan 0x000000601000-0x000000602000 /home/izbyshev/test/gcc/asan-exec-from-32bit/wrap-asan 0x0000f7dbd000-0x0000f7de2000 /lib64/ld-2.27.so 0x0000f7fe2000-0x0000f7fe3000 /lib64/ld-2.27.so 0x0000f7fe3000-0x0000f7fe4000 /lib64/ld-2.27.so 0x0000f7fe4000-0x0000f7fe5000 0x7fed9abff000-0x7fed9af54000 0x7fed9af54000-0x7fed9af6b000 /lib64/libgcc_s.so.1 [snip] 2) It doesn't seem to be great for security if an attacker always knows that ld.so is going to be mapped into the first 4GB in this case (the same thing happens for PIEs as well). The testcase: $ cat wrap.c int main(int argc, char *argv[]) { execvp(argv[1], &argv[1]); return 127; } $ gcc wrap.c -o wrap $ LD_SHOW_AUXV=1 ./wrap ./wrap true |& grep AT_BASE AT_BASE: 0x7f63b8309000 AT_BASE: 0x7faec143c000 AT_BASE: 0x7fbdb25fa000 $ gcc -m32 wrap.c -o wrap32 $ LD_SHOW_AUXV=1 ./wrap32 ./wrap true |& grep AT_BASE AT_BASE: 0xf7eff000 AT_BASE: 0xf7cee000 AT_BASE: 0x7f8b9774e000 Fixes: 1b028f784e8c ("x86/mm: Introduce mmap_compat_base() for 32-bit mmap()") Fixes: ada26481dfe6 ("x86/mm: Make in_compat_syscall() work during exec") Reported-by: Alexey Izbyshev Bisected-by: Alexander Monakov Investigated-by: Andy Lutomirski Signed-off-by: Dmitry Safonov Signed-off-by: Thomas Gleixner Reviewed-by: Cyrill Gorcunov Cc: Borislav Petkov Cc: Alexander Monakov Cc: Dmitry Safonov <0x7f454c46@gmail.com> Cc: stable@vger.kernel.org Cc: linux-mm@kvack.org Cc: Andy Lutomirski Cc: "H. Peter Anvin" Cc: Cyrill Gorcunov Cc: "Kirill A. Shutemov" Link: https://lkml.kernel.org/r/20180517233510.24996-1-dima@arista.com --- arch/x86/kernel/process_64.c | 1 + 1 file changed, 1 insertion(+) diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c index 4b100fe0f508..12bb445fb98d 100644 --- a/arch/x86/kernel/process_64.c +++ b/arch/x86/kernel/process_64.c @@ -542,6 +542,7 @@ void set_personality_64bit(void) clear_thread_flag(TIF_X32); /* Pretend that this comes from a 64bit execve */ task_pt_regs(current)->orig_ax = __NR_execve; + current_thread_info()->status &= ~TS_COMPAT; /* Ensure the corresponding mm is not marked. */ if (current->mm) -- 2.30.2