• Rob Gardner's avatar
    sparc64: fix FP corruption in user copy functions · a7c5724b
    Rob Gardner authored
    Short story: Exception handlers used by some copy_to_user() and
    copy_from_user() functions do not diligently clean up floating point
    register usage, and this can result in a user process seeing invalid
    values in floating point registers. This sometimes makes the process
    fail.
    
    Long story: Several cpu-specific (NG4, NG2, U1, U3) memcpy functions
    use floating point registers and VIS alignaddr/faligndata to
    accelerate data copying when source and dest addresses don't align
    well. Linux uses a lazy scheme for saving floating point registers; It
    is not done upon entering the kernel since it's a very expensive
    operation. Rather, it is done only when needed. If the kernel ends up
    not using FP regs during the course of some trap or system call, then
    it can return to user space without saving or restoring them.
    
    The various memcpy functions begin their FP code with VISEntry (or a
    variation thereof), which saves the FP regs. They conclude their FP
    code with VISExit (or a variation) which essentially marks the FP regs
    "clean", ie, they contain no unsaved values. fprs.FPRS_FEF is turned
    off so that a lazy restore will be triggered when/if the user process
    accesses floating point regs again.
    
    The bug is that the user copy variants of memcpy, copy_from_user() and
    copy_to_user(), employ an exception handling mechanism to detect faults
    when accessing user space addresses, and when this handler is invoked,
    an immediate return from the function is forced, and VISExit is not
    executed, thus leaving the fprs register in an indeterminate state,
    but often with fprs.FPRS_FEF set and one or more dirty bits. This
    results in a return to user space with invalid values in the FP regs,
    and since fprs.FPRS_FEF is on, no lazy restore occurs.
    
    This bug affects copy_to_user() and copy_from_user() for NG4, NG2,
    U3, and U1. All are fixed by using a new exception handler for those
    loads and stores that are done during the time between VISEnter and
    VISExit.
    
    n.b. In NG4memcpy, the problematic code can be triggered by a copy
    size greater than 128 bytes and an unaligned source address.  This bug
    is known to be the cause of random user process memory corruptions
    while perf is running with the callgraph option (ie, perf record -g).
    This occurs because perf uses copy_from_user() to read user stacks,
    and may fault when it follows a stack frame pointer off to an
    invalid page. Validation checks on the stack address just obscure
    the underlying problem.
    Signed-off-by: default avatarRob Gardner <rob.gardner@oracle.com>
    Signed-off-by: default avatarDave Aldridge <david.j.aldridge@oracle.com>
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    a7c5724b
U3memcpy.S 9.33 KB