diff options
Diffstat (limited to 'arch/x86/kernel/signal_32.c')
| -rw-r--r-- | arch/x86/kernel/signal_32.c | 62 |
1 files changed, 43 insertions, 19 deletions
diff --git a/arch/x86/kernel/signal_32.c b/arch/x86/kernel/signal_32.c index ef654530bf5a..98123ff10506 100644 --- a/arch/x86/kernel/signal_32.c +++ b/arch/x86/kernel/signal_32.c @@ -33,25 +33,55 @@ #include <asm/smap.h> #include <asm/gsseg.h> +/* + * The first GDT descriptor is reserved as 'NULL descriptor'. As bits 0 + * and 1 of a segment selector, i.e., the RPL bits, are NOT used to index + * GDT, selector values 0~3 all point to the NULL descriptor, thus values + * 0, 1, 2 and 3 are all valid NULL selector values. + * + * However IRET zeros ES, FS, GS, and DS segment registers if any of them + * is found to have any nonzero NULL selector value, which can be used by + * userspace in pre-FRED systems to spot any interrupt/exception by loading + * a nonzero NULL selector and waiting for it to become zero. Before FRED + * there was nothing software could do to prevent such an information leak. + * + * ERETU, the only legit instruction to return to userspace from kernel + * under FRED, by design does NOT zero any segment register to avoid this + * problem behavior. + * + * As such, leave NULL selector values 0~3 unchanged. + */ +static inline u16 fixup_rpl(u16 sel) +{ + return sel <= 3 ? sel : sel | 3; +} + #ifdef CONFIG_IA32_EMULATION #include <asm/unistd_32_ia32.h> static inline void reload_segments(struct sigcontext_32 *sc) { - unsigned int cur; + u16 cur; + /* + * Reload fs and gs if they have changed in the signal + * handler. This does not handle long fs/gs base changes in + * the handler, but does not clobber them at least in the + * normal case. + */ savesegment(gs, cur); - if ((sc->gs | 0x03) != cur) - load_gs_index(sc->gs | 0x03); + if (fixup_rpl(sc->gs) != cur) + load_gs_index(fixup_rpl(sc->gs)); savesegment(fs, cur); - if ((sc->fs | 0x03) != cur) - loadsegment(fs, sc->fs | 0x03); + if (fixup_rpl(sc->fs) != cur) + loadsegment(fs, fixup_rpl(sc->fs)); + savesegment(ds, cur); - if ((sc->ds | 0x03) != cur) - loadsegment(ds, sc->ds | 0x03); + if (fixup_rpl(sc->ds) != cur) + loadsegment(ds, fixup_rpl(sc->ds)); savesegment(es, cur); - if ((sc->es | 0x03) != cur) - loadsegment(es, sc->es | 0x03); + if (fixup_rpl(sc->es) != cur) + loadsegment(es, fixup_rpl(sc->es)); } #define sigset32_t compat_sigset_t @@ -105,18 +135,12 @@ static bool ia32_restore_sigcontext(struct pt_regs *regs, regs->orig_ax = -1; #ifdef CONFIG_IA32_EMULATION - /* - * Reload fs and gs if they have changed in the signal - * handler. This does not handle long fs/gs base changes in - * the handler, but does not clobber them at least in the - * normal case. - */ reload_segments(&sc); #else - loadsegment(gs, sc.gs); - regs->fs = sc.fs; - regs->es = sc.es; - regs->ds = sc.ds; + loadsegment(gs, fixup_rpl(sc.gs)); + regs->fs = fixup_rpl(sc.fs); + regs->es = fixup_rpl(sc.es); + regs->ds = fixup_rpl(sc.ds); #endif return fpu__restore_sig(compat_ptr(sc.fpstate), 1); |