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Unset LD_PRELOAD when ELF e_machine doesn't match. #17

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Merged
merged 2 commits into from
Feb 7, 2021
Merged

Unset LD_PRELOAD when ELF e_machine doesn't match. #17

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2e71dbd
Unset LD_PRELOAD when ELF e_machine doesn't match.
easyaspi314 Feb 3, 2021
57acdd2
Use ELFMAG macros.
easyaspi314 Feb 4, 2021
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82 changes: 55 additions & 27 deletions termux-exec.c
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Original file line number Diff line number Diff line change
Expand Up @@ -8,6 +8,7 @@
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <elf.h>

#ifndef TERMUX_BASE_DIR
# define TERMUX_BASE_DIR "/data/data/com.termux/files"
Expand All @@ -17,6 +18,18 @@
# define TERMUX_PREFIX "/data/data/com.termux/files/usr"
#endif

#ifdef __aarch64__
# define EM_NATIVE EM_AARCH64
#elif defined(__arm__) || defined(__thumb__)
# define EM_NATIVE EM_ARM
#elif defined(__x86_64__)
# define EM_NATIVE EM_X86_64
#elif defined(__i386__)
# define EM_NATIVE EM_386
#else
# error "unknown arch"
#endif

static const char* termux_rewrite_executable(const char* filename, char* buffer, int buffer_len)
{
strcpy(buffer, TERMUX_PREFIX "/bin/");
Expand All @@ -30,6 +43,29 @@ static const char* termux_rewrite_executable(const char* filename, char* buffer,
return filename;
}

static char*const * remove_ld_preload(char*const * envp)
{
for (int i = 0; envp[i] != NULL; i++) {
if (strstr(envp[i], "LD_PRELOAD=") == envp[i]) {
int env_length = 0;
while (envp[env_length] != NULL) env_length++;

char** new_envp = malloc(sizeof(char*) * env_length);
int new_envp_idx = 0;
int old_envp_idx = 0;
while (old_envp_idx < env_length) {
if (old_envp_idx != i) {
new_envp[new_envp_idx++] = envp[old_envp_idx];
}
old_envp_idx++;
}
new_envp[env_length] = NULL;
return new_envp;
}
}
return envp;
}

int execve(const char* filename, char* const* argv, char *const envp[])
{
bool android_10_debug = getenv("TERMUX_ANDROID10_DEBUG") != NULL;
Expand All @@ -56,12 +92,23 @@ int execve(const char* filename, char* const* argv, char *const envp[])

// execve(2): "A maximum line length of 127 characters is allowed
// for the first line in a #! executable shell script."
char shebang[128];
ssize_t read_bytes = read(fd, shebang, sizeof(shebang) - 1);
if (read_bytes < 5 || !(shebang[0] == '#' && shebang[1] == '!')) goto final;
char header[128];
ssize_t read_bytes = read(fd, header, sizeof(header) - 1);

// If we are executing a non-native ELF file, unset LD_PRELOAD.
// This avoids CANNOT LINK EXECUTABLE errors when running 32-bit code
// on 64-bit.
if (read_bytes >= 20 && !memcmp(header, ELFMAG, SELFMAG)) {
Elf32_Ehdr* ehdr = (Elf32_Ehdr*)header;
if (ehdr->e_machine != EM_NATIVE) {
envp = remove_ld_preload(envp);
}
goto final;
}
if (read_bytes < 5 || !(header[0] == '#' && header[1] == '!')) goto final;

shebang[read_bytes] = 0;
char* newline_location = strchr(shebang, '\n');
header[read_bytes] = 0;
char* newline_location = strchr(header, '\n');
if (newline_location == NULL) goto final;

// Strip whitespace at end of shebang:
Expand All @@ -71,7 +118,7 @@ int execve(const char* filename, char* const* argv, char *const envp[])
*newline_location = 0;

// Skip whitespace to find interpreter start:
char* interpreter = shebang + 2;
char* interpreter = header + 2;
while (*interpreter == ' ') interpreter++;
if (interpreter == newline_location) goto final;

Expand Down Expand Up @@ -135,27 +182,8 @@ int execve(const char* filename, char* const* argv, char *const envp[])
}
new_argv[orig_argv_count + 1] = NULL;
argv = (char**) new_argv;

// Remove LD_PRELOAD environment variable when wrapping in proot:
for (int i = 0; envp[i] != NULL; i++) {
if (strstr(envp[i], "LD_PRELOAD=") == envp[i]) {
int env_length = 0;
while (envp[env_length] != NULL) env_length++;

char** new_envp = malloc(sizeof(char*) * env_length);
int new_envp_idx = 0;
int old_envp_idx = 0;
while (old_envp_idx < env_length) {
if (old_envp_idx != i) {
new_envp[new_envp_idx++] = envp[old_envp_idx];
}
old_envp_idx++;
}
new_envp[env_length] = NULL;
envp = new_envp;
break;
}
}
// Remove LD_PRELOAD environment variable when wrapping in proot
envp = remove_ld_preload(envp);
}
} else {
errno = 0;
Expand Down