From: Laurent ALFONSI <laurent.alfo...@st.com> Note that the support for the command-line argument requires:
1. add the new field "uint8_t rand_bytes[16]" to "struct image_info" since only the variable "info" lives both in main() and in create_elf_tables() 2. write a dedicated parser to convert the command-line to fill rand_bytes[] These two steps aren't really hard to achieve but I finally think they are a little bit overkill regarding the purpose of these 16 bytes. Maybe we could always fill the 16 bytes pointed to by AT_RANDOM with zero if we really want to get reproducibility. Regards, Cédric. 8<----8<----8<----8<----8<----8<----8<----8<----8<----8<----8<----8<---- The dynamic linker from the GNU C library v2.10+ uses the ELF auxiliary vector AT_RANDOM [1] as a pointer to 16 bytes with random values to initialize the stack protection mechanism. Technically the emulated GNU dynamic linker crashes due to a NULL pointer derefencement if it is built with stack protection enabled and if AT_RANDOM is not defined by the QEMU ELF loader. [1] This ELF auxiliary vector was introduced in Linux v2.6.29. This patch can be tested with the code above: #include <elf.h> /* Elf*_auxv_t, AT_RANDOM, */ #include <stdio.h> /* printf(3), */ #include <stdlib.h> /* exit(3), EXIT_*, */ #include <stdint.h> /* uint8_t, */ #include <string.h> /* memcpy(3), */ #if defined(__LP64__) || defined(__ILP64__) || defined(__LLP64__) # define Elf_auxv_t Elf64_auxv_t #else # define Elf_auxv_t Elf32_auxv_t #endif main(int argc, char* argv[], char* envp[]) { Elf_auxv_t *auxv; /* *envp = NULL marks end of envp. */ while (*envp++ != NULL); /* auxv->a_type = AT_NULL marks the end of auxv. */ for (auxv = (Elf_auxv_t *)envp; auxv->a_type != AT_NULL; auxv++) { if (auxv->a_type == AT_RANDOM) { int i; uint8_t rand_bytes[16]; printf("AT_RANDOM is: 0x%x\n", auxv->a_un.a_val); memcpy(rand_bytes, (const uint8_t *)auxv->a_un.a_val, sizeof(rand_bytes)); printf("it points to: "); for (i = 0; i < 16; i++) { printf("0x%02x ", rand_bytes[i]); } printf("\n"); exit(EXIT_SUCCESS); } } exit(EXIT_FAILURE); } Changes introduced in v2 and v3: * Fix typos + thinko (AT_RANDOM is used for stack canary, not for ASLR) * AT_RANDOM points to 16 random bytes stored inside the user stack. * Add a small test program. Signed-off-by: Cédric VINCENT <cedric.vinc...@st.com> Signed-off-by: Laurent ALFONSI <laurent.alfo...@st.com> --- linux-user/elfload.c | 21 ++++++++++++++++++++- 1 files changed, 20 insertions(+), 1 deletions(-) diff --git a/linux-user/elfload.c b/linux-user/elfload.c index dcfeb7a..23c69d9 100644 --- a/linux-user/elfload.c +++ b/linux-user/elfload.c @@ -927,7 +927,7 @@ struct exec #define TARGET_ELF_PAGESTART(_v) ((_v) & ~(unsigned long)(TARGET_ELF_EXEC_PAGESIZE-1)) #define TARGET_ELF_PAGEOFFSET(_v) ((_v) & (TARGET_ELF_EXEC_PAGESIZE-1)) -#define DLINFO_ITEMS 12 +#define DLINFO_ITEMS 13 static inline void memcpy_fromfs(void * to, const void * from, unsigned long n) { @@ -1202,6 +1202,9 @@ static abi_ulong create_elf_tables(abi_ulong p, int argc, int envc, { abi_ulong sp; int size; + int i; + abi_ulong u_rand_bytes; + uint8_t k_rand_bytes[16]; abi_ulong u_platform; const char *k_platform; const int n = sizeof(elf_addr_t); @@ -1231,6 +1234,20 @@ static abi_ulong create_elf_tables(abi_ulong p, int argc, int envc, /* FIXME - check return value of memcpy_to_target() for failure */ memcpy_to_target(sp, k_platform, len); } + + /* + * Generate 16 random bytes for userspace PRNG seeding (not + * cryptically secure but it's not the aim of QEMU). + */ + srand((unsigned int) time(NULL)); + for (i = 0; i < 16; i++) { + k_rand_bytes[i] = rand(); + } + sp -= 16; + u_rand_bytes = sp; + /* FIXME - check return value of memcpy_to_target() for failure */ + memcpy_to_target(sp, k_rand_bytes, 16); + /* * Force 16 byte _final_ alignment here for generality. */ @@ -1271,6 +1288,8 @@ static abi_ulong create_elf_tables(abi_ulong p, int argc, int envc, NEW_AUX_ENT(AT_EGID, (abi_ulong) getegid()); NEW_AUX_ENT(AT_HWCAP, (abi_ulong) ELF_HWCAP); NEW_AUX_ENT(AT_CLKTCK, (abi_ulong) sysconf(_SC_CLK_TCK)); + NEW_AUX_ENT(AT_RANDOM, (abi_ulong) u_rand_bytes); + if (k_platform) NEW_AUX_ENT(AT_PLATFORM, u_platform); #ifdef ARCH_DLINFO -- 1.7.5.1