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Source: https://bugs.chromium.org/p/project-zero/issues/detail?id=990 The following function (and variations on the same code) is used to write to files from kernel code in various touchscreen drivers. This copy is from RefCode_CustomerImplementation.c - I'm unsure which copy is actually being used on the LG G4, but I can trigger the vulnerability. A function with the same issues exists as "write_file" in several files. int _write_log(char *filename, char *data) { struct file *file; loff_t pos = 0; int flags; char *fname = "/data/logger/touch_self_test.txt"; char *fname_normal_boot = "/sdcard/touch_self_test.txt"; char *fname_mfts_folder = "/data/logger/touch_self_test_mfts_folder.txt"; char *fname_mfts_flat = "/data/logger/touch_self_test_mfts_flat.txt"; char *fname_mfts_curved = "/data/logger/touch_self_test_mfts_curved.txt"; int cap_file_exist = 0; if (f54_window_crack || f54_window_crack_check_mode == 0) { mm_segment_t old_fs = get_fs(); set_fs(KERNEL_DS); flags = O_WRONLY | O_CREAT; if (filename == NULL) { flags |= O_APPEND; switch (mfts_mode) { case 0: if (factory_boot) filename = fname; else filename = fname_normal_boot; break; case 1: filename = fname_mfts_folder; break; case 2: filename = fname_mfts_flat; break; case 3: filename = fname_mfts_curved; break; default: TOUCH_I("%s : not support mfts_mode\n", __func__); break; } } else { cap_file_exist = 1; } if (filename) { file = filp_open(filename, flags, 0666); sys_chmod(filename, 0666); } else { TOUCH_E("%s : filename is NULL, can not open FILE\n", __func__); return -1; } if (IS_ERR(file)) { TOUCH_I("%s : ERR(%ld)Open file error [%s]\n", __func__, PTR_ERR(file), filename); set_fs(old_fs); return PTR_ERR(file); } vfs_write(file, data, strlen(data), &pos); filp_close(file, 0); set_fs(old_fs); log_file_size_check(filename); } return cap_file_exist; } int write_log(char *filename, char *data) { return _write_log(filename, data); } This code is setting KERNEL_DS, and there is a code-path in which it does not restore USER_DS before returning (when mfts_mode is outside the range [0, 3] and the filename argument is NULL). This can be triggered by first writing to the sysfs node /sys/devices/virtual/input/lge_touch/mfts and then reading from the sysfs node /sys/devices/virtual/input/lge_touch/sd. (root needed to write to mfts node). Once the kernel has returned control to userland with KERNEL_DS set, userland can simply read/write from arbitrary kernel addresses. See attached for a working exploit for the LG G4, which when run as root will disable selinux enforcement. Proof of Concept: https://gitlab.com/exploit-database/exploitdb-bin-sploits/-/raw/main/bin-sploits/41353.zip |
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