Messages in this thread Patch in this message | | | Date | Tue, 11 May 1999 15:57:29 +0200 | From | Pavel Machek <> | Subject | [PATCH] capabilities done right |
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Hi!
This time even with documentation :-). Ok, I changed parsing so that it walks all notes in all segments and ands the results. I added documentation. Please comment,
Pavel PS: Alan, please take a look. Does it look like capable of getting into 2.3.1? [I do not want to ask Linus just now, and you are next god after him ;-)]
PPS: Please edit to: field after reply...
--- /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/fs/binfmt_elf.c Fri Mar 26 17:46:23 1999 +++ linux/fs/binfmt_elf.c Tue May 11 15:49:19 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++; } --- clean/include/linux/elf.h Thu Jun 25 17:38:14 1998 +++ linux/include/linux/elf.h Tue May 11 13:03:45 1999 @@ -496,6 +496,39 @@ 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; + Elf32_Word effective; + Elf32_Word effective1; + Elf32_Word effective2; + Elf32_Word effective3; + Elf32_Word permitted; + Elf32_Word permitted1; + Elf32_Word permitted2; + Elf32_Word permitted3; + Elf32_Word inheritable; + Elf32_Word inheritable1; + Elf32_Word inheritable2; + Elf32_Word inheritable3; + Elf32_Word known; + Elf32_Word known1; + Elf32_Word known2; + Elf32_Word known3; +}; + +struct elf_capabilities_note { + Elf32_Nhdr nhdr; + __u32 note_signature; /* == "CAPS" */ + struct elf_capabilities cap; +}; + #if ELF_CLASS == ELFCLASS32 extern Elf32_Dyn _DYNAMIC [];
-- 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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