Messages in this thread Patch in this message | | | Date | Fri, 14 May 1999 23:33:53 +0200 | From | Pavel Machek <> | Subject | Capabilities done right [diff against 2.3.1] |
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Hi!
Next try with capabilities, this time against 2.3.1. Patch is completely safe and should significantly enhance system security. It is completely backward compatible: ie. no semantics change. Capabilities are implemented using elf notes (and this version parses notes correctly). Software exists for adding capabilities at runtime, so you don't even require a recompile.
This time I removed "FLE" hack, which was maybe too dirty. Please apply, Pavel
PS: Pretty nice page exists at http://atrey.karlin.mff.cuni.cz/~pavel/elfcap.html; and this time it really is there.
--- /dev/null Tue Jul 21 02:45:36 1998 +++ linux/Documentation/capabilities.txt Tue May 11 14:17:39 1999 @@ -0,0 +1,31 @@ +Elf capabilities hack +===================== + +From now on, there's support for capabilities in elf executable. Elf +executable now may contain "capabilities header", telling which +capabilities should be dropped on exec. This can not hurt: lowering +capabilities is not priviledged operation, and executable could do it +itself at beggining of main. + +Doing it in exec() time has certain advantages, through: you can +easily look and what capabilities are in use by what program and you +can set capabilities for existing executables without need to +recompile. + +What can elfcap do: + +* mask inheritable, permitted and effective sets by arbitrary mask + +* set euid back to ruid + +Along with existing setuid mechanism, this hack can be used to grant +subset of capabilities to executables. For example currently ping has +to be setuid0. With elfcap, ping still will be setuid0, but most of +its capabilities will be dropped at exec() time, so breaking into ping +will allow attacker to generate arbitrary packets to network, but +nothing more. + +For more info & utility programs, look at +http://atrey.karlin.mff.cuni.cz/~pavel/elfcap.html. + + pavel@ucw.cz --- clean/include/linux/elf.h Thu Jun 25 17:38:14 1998 +++ linux/include/linux/elf.h Tue May 11 15:58:41 1999 @@ -496,6 +496,27 @@ Elf32_Word n_type; /* Content type */ } Elf64_Nhdr; +/* Capabilities support + */ +struct elf_capabilities { + Elf32_Word signature; + Elf32_Word version; /* Currently 0; this is so that you can append on the end painlessly */ + Elf32_Word flags; +#define ECF_MAKE_EUID_UID 1 +#define ECF_MAKE_EUID_XUID 2 + Elf32_Word xuid; /* We want our set 128bit for future expansion */ + Elf32_Word effective, effective1, effective2, effective3; + Elf32_Word permitted, permitted1, permitted2, permitted3; + Elf32_Word inheritable, inheritable1, inheritable2, inheritable3; + Elf32_Word known, known1, known2, known3; +}; + +struct elf_capabilities_note { + Elf32_Nhdr nhdr; + __u32 note_signature; /* == "CAPS" */ + struct elf_capabilities cap; +}; + #if ELF_CLASS == ELFCLASS32 extern Elf32_Dyn _DYNAMIC []; --- clean/fs/binfmt_elf.c Thu May 13 22:41:36 1999 +++ linux/fs/binfmt_elf.c Thu May 13 22:32:54 1999 @@ -7,6 +7,7 @@ * Tools". * * Copyright 1993, 1994: Eric Youngdale (ericy@cais.com). + * Capabilities copyright 1999 Pavel Machek (pavel@ucw.cz). */ #include <linux/module.h> @@ -387,6 +388,26 @@ return elf_entry; } +static void +restrict( struct linux_binprm * bprm, struct elf_capabilities *cap ) +{ + if (cap->signature != 0xca5ab1e) + return; + + /* I do not check versions... That is because current version + is 0 and I expect all changes to be backward - compabtible */ + if (cap->flags & ECF_MAKE_EUID_UID) /* You may want to loose owner's uid */ + bprm->e_uid = current->uid; + if ((!bprm->e_uid) && (cap->flags & ECF_MAKE_EUID_XUID)) + bprm->e_uid = cap->xuid; /* We only honour random uid changes for root */ + cap_mask( bprm->cap_effective, cap->effective ); + cap_mask( bprm->cap_permitted, cap->permitted ); + cap_mask( bprm->cap_inheritable, cap->inheritable ); + + printk( KERN_DEBUG "Now: uid = %d, effective = %x, permitted = %x, inheritable = %x\n", bprm->e_uid, bprm->cap_effective, bprm->cap_permitted, bprm->cap_inheritable ); +} + + /* * These are the functions used to load ELF style executables and shared * libraries. There is no binary dependent code anywhere else. @@ -396,6 +417,7 @@ #define INTERPRETER_AOUT 1 #define INTERPRETER_ELF 2 +#define roundup(x, y) ((((x)+((y)-1))/(y))*(y)) static inline int do_load_elf_binary(struct linux_binprm * bprm, struct pt_regs * regs) @@ -473,6 +498,22 @@ end_data = 0; for (i = 0; i < elf_ex.e_phnum; i++) { + if (elf_ppnt->p_type == PT_NOTE) { + struct elf_capabilities_note note; + int offset = elf_ppnt->p_offset; + int maxoffset = offset + elf_ppnt->p_filesz; + + while (offset <= (maxoffset - sizeof(note))) { + int retval; + retval = read_exec(bprm->dentry, offset, (void *) ¬e, + sizeof(note), 1); + if (retval != sizeof(note)) + goto skip; + if (note.note_signature == be32_to_cpu(0x43415053)) /* "CAPS" */ + restrict(bprm, ¬e.cap); + offset += sizeof(Elf32_Nhdr) + roundup(note.nhdr.n_namesz, 4) + roundup(note.nhdr.n_descsz, 4); + } + } if (elf_ppnt->p_type == PT_INTERP) { retval = -EINVAL; if (elf_interpreter) @@ -533,6 +574,7 @@ interp_ex = *((struct exec *) bprm->buf); interp_elf_ex = *((struct elfhdr *) bprm->buf); } + skip: elf_ppnt++; } -- I'm really pavel@ucw.cz. Look at http://195.113.31.123/~pavel. Pavel Hi! I'm a .signature virus! Copy me into your ~/.signature to help me spread!
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